I have witnessed similar at Taunton, a one-day game against Warwickshire comes to mind - usual story; football fans becoming aggressive in the late afternoon/early evening after too many beers. I also recall a Somerset/Glos game several years ago at Bath that was particularly rowdy, with the Police having to break up fights between City and Rovers fans and ejecting several.
These were all alcohol related, however, and I think Tomarse referred recently to a T20 Glamorgan game at Taunton when the Glamorgan fans were 'allegedly' fuelled by coke - a disappointing descent.
I have just seen Phantom's post, so it may have been him rather than Tomarse who mentioned the Glamorgan game.