Why were the leaders of the Soviet Union worried when Solidarity was
set up in 1980-1981?

Not a direct answer to your question but detailed and interesting:

"Polish-Soviet Relations 1944-1989: The Limits of Autonomy" by Andrzej Paczkowski:
http://www.columbia.edu/cu/sipa/REGIONAL/ECE/vol6no1/paczkowski.pdf


"Poland, 1980-1982: Internal Crisis, International Dimensions" 8-10 November 1997:
http://www.columbia.edu/cu/sipa/REGIONAL/ECE/vol2no1/jaruzels.pdf

Do these sentences answer your question?

"In the sixteen months following its initial strike, Solidarity waged
a difficult campaign to realize the letter and spirit of the Gdansk
Agreement. This struggle fostered an openness unprecedented in a
communist East European society.
... 
Meanwhile, the persistence of Solidarity prompted furious objections
from Moscow and other Comecon members, putting Poland under constant
threat of invasion by its Warsaw Pact allies. This was the first time
a ruling communist regime had accepted organizations completely beyond
the regime's control. It was also the first time an overwhelming
majority of the workers under such a regime were openly loyal to an
organization fundamentally opposed to everything for which the party
stood."
http://www.country-studies.com/poland/the-birth-of-solidarity.html

"Poland country studies":
http://www.country-studies.com/poland/

http://www.country-studies.com/

I have a BA in History and find this of enormous interest:
The leaders of the USSR feared Solidarity because Poland was behind
the Iron Curtain and part of the Soviet 'empire' and the USSR brooked
very little dissent within their empire.  They had invaded Hungary in
1956 and Czech in 1968 to stop similar independence drives.  To many,
including Carter and Reagan, it looked like the USSR was going to send
troops in to stop Solidarity and the drive for greater ind. from
Soviet control.  Additionally, Solidarity had close links with the
Polish Catholic Church, which irked the Soviets.  Brezhnev had already
invaded Afghanistan in Dec. of 79 and it looked the same in Poland. 
Thankfully, no Soviet or Warsaw Pact troops invaded.  The Soviets had
Polish General Jaruzelski do their dirty work for them: arrested
Walesa, shut down the party, and arrested a ton of other ppl.
Hope this helps ... let me know if I can help with another angle