I just got permanently banned from Adwords a month ago too.
It took me 5 years to finally reach the minimum payout, and they banned my account right before the 2nd payout. I did not do any click fraud.
The only other explanation I can think of is "click bombing" but I did not have logs longer than a week and did not see anything unusual.
It is infuriating that not only did they steal $100 from me (it was stolen from me because I could've used the other advertisers I am using now during those months), but that I am banned without an explanation. And they have the gall to ask me to try out Adwords with a $100 coupon.