Why is sound on my Android suddenly increasing?
It could most likely be a bug. I suggest you upgrade your Android via Kies or manually.

Galaxy Nexus is known to have a similar software issue with volume: http://techie-buzz.com/mobile-news/galaxy-nexus-to-get-a-software-update-to-fix-the-volume-bug.html Perhaps it could be the same with your S2, especially since it is on older Android version.