I am seeing a degradation of broad match traffic in Bing, particularly if a bid is raised. So, if you have an ad that is converting well at a given price and you have some wiggle room on the CPC, you may find that the quality of traffic goes down if you raise the bid. The greater bid may cause you to have shittier traffic.
Does this shed some light ?
http://www.rimmkaufman.com/blog/dropping-bids-on-bing-be-ready/23082010/