I think it's because someone tighthened the screws a tad bit too tight, it happened to my UV777bk as well. But I hated the cheapo looking plastic mirror in the very first place! The fact that it cracked gave me a good excuse to get a custom one....A FULL STAINLESS STEEL ONE BABEEE!
ROCK!
You can get one done, not necessarily at a pickguard place, I got mine done at a local custom sign maker!

With a turnout time of one a half weeks and a price of about 50 US Dollars and the fact that my UV looks really nice...who's complaining?
Chris.