Windows losing focus

Since last nights update my windows in Windows loses focus pretty frequently.
Looking at the task manager it seems that every time it happens is when wermgr.exe starts.
Investigating a little further I remembered that I installed a service that crashes once a minute (I tested the exception handling of the service).
It wasn't running before, but started when I restarted the computer after the update.
So, either wermgr steals focus whenever it starts or my service did.
Anyway, problem solved.