It looks like when playing a skeleton BG the game freezes every 2-5 minutes. I did not see it freezing with other non-skele BG I played. Seems there is no connection to specific champions or abilities, it freezes at random times during my turns.
I just tried to reproduce this and playing a skeleton BG not once did the game crash. Make sure all your computer's drivers are up to date. Unfortunately, without more information there isn't much we can do.
Here is the BG: http://poxbase.com/#!/deck/chMc2fcaNChNC69ChXChVC4bcuBc4nswS12Sns4fS2_r3lrTRvr8E2B The same freezing problem was also happening with slightly different skele champs. Here is screens:
