If you have FileMaker Advanced you can open up the script debugger and see what script is calling it. It's most likely a script trigger if you think it's 'random'. So maybe one or more fields has a script trigger attached to it, or the layout has a script trigger attached to it.
The next time I see this error, I will open the script debugger window and report my findings. Thanks for the help.
I exported all the scripts and did a search for "pause".
There was one occurrence of an "Enter Browse Mode [Pause: On]" step in a script that prints laboratory results to a PDF file. I changed the parameter to [Pause: Off], saved the script, and that seems to have fixed the problem.
Thanks for the help. I will keep an eye on things to make sure it really is fixed.