I was pulled over for speeding. While the officer was still in his car
I got out of mine with my driving licence in hand. He yelled at me
"get back in the car" I am interested to know the theory behind this
instruction. Surely with me out of the vehicle he can see I dont have
a gun and that I will not speed off in my car. What gives?

Hello.

Here's a good explanation from the Walpole Police Department:

"Stay Seated-  First, and most importantly, put your vehicle in park
and stay seated.
-Why?-  Putting your vehicle in park is simply a safety precaution, it
also lets the officer know that you are not planning on going
anywhere.  The most crucial thing to remember throughout the entire
stop is that the officer does not know who you are, or what is in your
vehicle.  You may know that you are a law abiding citizen with every
intention of cooperating fully, but the officer does not have the
luxury of this information.  The rear of a car looks the same,
regardless if there is a sweet grandma driving with a trunk full of
groceries, or a dangerous criminal driving with a trunk full of
contraband.  Officers ask that you stay in your vehicle so that you
are not struck by a passing car.  On the same token, they do not know
if you are getting out to run, or fight, or tell us you are in a
hurry.  Either way, it is crucial that you stay in your vehicle during
the entire stop, for your safety and that of the officer."
http://www.walpolepd.com/traffic_safety.htm


From Findlaw.com:

"Stay in your car. While a stopping police officer has the right to
ask you to step out of your car, do not do so unless and until you are
asked to. If you jump out of your car as the police officer is
approaching or is standing next to your car, you will likely give him
reason to suspect that you are trying to flee or attack."
http://public.findlaw.com/traffic_violations/life_events/le4_4tips.html

--------
search strategy:
"stay in your" "pulled over"

I hope this helps.
From SmartMotorist.com:

"Do I stay in or get out of my car? Most officers prefer (or they
should prefer) that you stay in your car when you are stopped. If they
want you to get out, they will ask you to. From an officer's
standpoint, many criminals will exit their car to try to prevent the
officer from seeing what they have in there with them. Staying in the
car also allows the officer to focus his/her attention on the
passenger compartment alone and not it plus you if you get out. If you
do get out and the officers tells you to get back in, do not be
offended...he/she is just trying to their job safely."
http://www.smartmotorist.com/law/law.htm


From Nolo.com:

"Many police officers prefer that you stay in your car, and will tell
you to stay there if you start to get out. If this happens, obviously
you should cooperate. If you get out of the car against the officer's
orders, don't be surprised to see a gun pointing at you. Cops are
trained to expect the worst. When you get out of your car, they may
assume you're about to pull a weapon or attempt to flee. So, don't
panic, and make it apparent to the officer that you intend no harm."
http://www.nolo.com/lawcenter/ency/article.cfm/ObjectID/DF83EA05-BCBE-4B8A-8A42F1815BF6BF81/catID/CF015A63-6B69-4EED-A34B6F4035C8BE0E

Fast and good answer. Thanks.

Those are good answers, and the first one touchs on an important note
which can be summed up with an all important word in the police
vocabulary - liability.  When a police officer stops a vehicle, he has
exerted minimal 'force' to make that stop, and although not an arrest,
it is a detainment, which is why 'probable cause' is need to make the
stop. (4th Ammendment - search and seizure)

Now consider, your driving down the road, and you stop for a turtle in
the road. You get out of your car, and splat!! you get creamed by an
inattentive motorist.  That is your fault.

To the contrary, you're driving down the road, and you hear a siren,
and looking up you see Johnnie Law behind you.  So you pull over and
wondering what is taking the officer so long, decide to get out and
SPLAT!!  Well that could be construded to be the officers fault
(Considering the officer had a reasonable amount of time to see you
get out of the car and approach, and he failed to advise you to get
back into the car) When the officer stopped you, or seized your
ablitiy to continue down the road, it is believed that he is now
responsible for you.

In today's lawsuit happy world, everyone from the officer, to his
Sgt., to his Chief, to the Council, to the Mayor would be sued,
because the officer, who is trained, and should have known better
didn't look out for you, the law-abidding citizens safety.

So let's say, Mr. Not-so-law-abiding is legally pulled over, and being
who he is, decides to run, whether it be by exiting the car, or
stomping the gas, he has just committed a crime (normally a traffic
infraction) of 'fleeing and eluding'.  And in the course of running,
Mr. Not-so-law-abiding is creamed, SPALT!!  No foul...it was his
choice to run and break the law, and the officer can not be held
accountable.

I hope I have made this interesting in that I have given a different
point of view.  Me personally, I'm more concerned about doing my job
professionally, correctly, considering the safety of all those
involved, then I am about someone fleeing or attempting to hide
contraband from my view.

Thanks again juggler-ga for your answer.
Many thanks naysayeri3 for an officers observations and the 
down to earth illustration. I had'nt realised it was for my safety
I was to remain in the car.