You could check for a crash log at the same time. They're in Settings–>Privacy–>Analytics–>Analytics Data. They'll be named with the app name, MidiFire. Ignore those with wakeups_reource or cpu_resource in the name. This would indicate whether the app has actually crashed (or been killed), or whether it just stopped communicating. If you're able to read it, look for the lines with the words "exception" and "ktriageinfo" near the beginning. They may give clues to the cause. You could copy those two lines and post them here. The rest of the log is basically gibberish. If you find a log, you could send one to nic.
Edit: Have you checked the "background idle timeout" in the app Setup? You may just have had a quiet interval causing the app to decide to quit. You could set it to "never" to prevent it interfering. Just be sure to force-quit the app when you're done.
There's a separate log file for each event. So you'll see a log for each crash, with the date/time in the filename. They're retained for a week. And if you sync the iPad with a computer, using Finder or iTunes, the logs are transferred to the computer, where they'll be saved forever. Takes a little digging to find them, but they are there.
Your first post was Dec. 4, so the log should still be there. Take a look.