Google surprised AdWords users in May this year with their announcement that AdWords ad rotation would automatically stop and favour only the ad with highest CTR, after the ads were active for 30 days.

Google’s ad rotation settings will switch from ‘rotate’ to favor the ad with highest CTR, unless you take action.  If you don’t want to opt out, and you want to restore even rotation on your ads after the 90 day period, it is not a huge problem.