There can be SO many different reasons for drops like this. Impossible to say without seeing the listings. You could be suffering from a ranking penalty for unwittingly breaking some of the Google Places unwritten or misunderstood rules or one of the algo changes could have affected you.
But I'll tell you a couple common issues it could be.
One of the most common reasons is that - there are 2 totally different (almost completely opposite) local algos. You COULD have been ranking well in the new "blended" algo and then Google switched some of your markets back to the "Old Places" algo. Or vice versa. One algo is more dependent on your on-site SEO factors and the other is more weighted toward how many citations and reviews you have. So you can easily drop OR increase several pages overnight if you are only well optimized for one algo BUT NOT both!
Did you notice if those markets USED to show the new big merged listings, but now are showing the small 2 line 7 pack style listings? Or vice versa? Did the TYPE of SERPs layout style change? If you've noticed that, it will tell you which algo is at play.
Also a current bug is causing rankings to drop lately. You can check to see if you may have gotten bitten. Are your Place pages missing descriptions, hours or more details even though that data is listed in the dashboard?
That bug is affecting lots of users right now and often causes an instant ranking drop. I talked to the Google Places Community Manager about it and my Places rep tried to help fix it for one of my clients. More and more are reporting the problem on the GP forum and I am still trying to get Google to fix, but so far no luck. :-(
My bank client had an issue because it had the same phone number for every branch (700+) among other issues such as all-caps (the bank has an acronym as a name). Another issue to check is using a keyphrase as the business name (where it isn't the business name). One of my listings was suspended for that reason.
Do you know if the traffic you were getting was from the organic listings or from the local results (the 7 pack or listings with drop-points next to them). My guess is that you were getting organic traffic and that has been hit by Panda in early april and taken a steady decline since then.
If that's the case then you need to look at differentiating those landing pages with useful stuff - not just fluff (maybe testimonials, details of staff/experience in each location...)
Another thing to do is get stuck into social; Google needs to see people liking your services, it does not have many ways to decide who's best and they are getting serious about social.