Log into iTunes connect and look for crash reports.
Take these reports very seriously. For every complaint you get, there are 10 times that many people having the same problem. This sort of this can drive the ratings and reviews down such that they never recover.
Is there any crash log data available on your iTunesConnect portal? While you probably won’t be able to correlate that back to individual customers, if logs are available you’ll at least get a stack trace as well as OS versions of the devices reporting crashes.
The first thing I usually tell folks is to reboot their device, and also to query them about what hardware they have and what level of OS they have installed. If you have a support website, standardizing the feedback you get along those lines is very useful. Things like does the app run for you? Does it crash in the same place, or randomly? What were you doing at the moment it crashed?
If customers feel like helping rather than demanding an immediate refund, this is a good visual explanation to help them email you their specific crash logs:
It may take crash logs awhile to show up. I think the user has to sync with a desktop before they are sent to Apple.
I doubt very much it’s too many apps.
I had a similar issue where my app had too much audio. It worked most of the time but crashed on some devices. The behavior wasn’t consistent. After I replaced the audio with lower quality files, I stopped getting complaints.
Of course, my app received lots of low star reviews and some negative comments about crashing and sales have never recovered.
I have another, very similar app, with less functionality but no crash reports which constantly outsells the low rating one.