Archaeologists are always digging up burried building foundations of
ancient cities. How is it that they get burried in the first place? I
mean, after the building falls down or whatever, over time does that
much dirt get deposited?  A satisfactory answer would be a summary and
link to a couple articles.

This may be of interest to you:

http://www.archaeological.org/webinfo.php?page=10271#faq8

The brief FAQ on Pinkfreud's link is a good summary.  In Europe there
are many churchs that now have a couple of steps down to an entrance
because the area around the building has been filled in in the course
of only a few hundred years.  When the paving gets bad (cobblestones
or whatever), it's easier just to add a new layer, and, of course, if
a building falls down and is not rebuilt, a pile of rubble will
attract natural and other debris.  In Rome, it really is intriguing 
how deep some excavated areas are.
ON the other hand, "archaeologists are always digging" 'cause that's
where they can find things.

In Israel and much Near Eastern archaeology, there is what is known as
"tels."  A tel from far away looks like a large hill or even small
mountain rising from a plain, but is in fact, an archaeological site. 
What happens is that a city gets built.  Perhaps it gets conquered and
burned, or simply abandoned for whatever reason.  Later, another group
comes and, instead of tearing down, simply builds a new city over the
existing one.  This process could literally happen  dozens of times. 
Eventually, you could have a mound that is several hundred feet tall
and takes several hours to walk to the top of!  Archaeologists will
then dig these sites one layer at a time (although sometimes they
can't help but get a sneak preview of what is to come:))

I just read in the paper today about a chap in Italy who was using
Google Earth to look around his neighborhood and discovered something
unusual in a field.  The archaeologists are now digging.

archaeologists are alsways studying the factors that create/form sites
known  as site formation processes (rather dull really!) the simple
answer is erosion and deposition, A ruinous building will decay fairly
predictably, firstly the roof will be likely to fall in (or the walls
fall out combined with the roof falling in), the actions of animals,
plants, mechanical, chemical weathering etc. as plants colonise they
trap soil and eventually over time (im keeping it brief to spare you
the hunded or so years of quite boring goings on) soil builds up. its
basic geography.
http://www.sscnet.ucla.edu/ioa/backdirt/spring99/mink.html
an example is the formation of 'black dirt' on the coast of Alaska.