EKF divergence?

I had a weird incident a few days ago. I was prepping an older airframe that had a few dozen flights on it. I powered it up on a shop battery with no props to get GPS and make sure everything was functional through arming. Everything worked fine. I un-powered, installed props, installed a flight battery and powered it back up. When I looked at Mission Planner, the horizon had an angle that looked larger than the quad was resting at. I carried the quad back to the GCS to make sure the horizon responded to motions - it did. I set it down, and then was distracted for a few minutes talking with colleagues. When I looked back at the horizon in MP, all I saw was sky, even though the copter was sitting on pretty level terrain. The horizon still showed rates proportional to rates I moved the quad, but it was off by about 90 degrees. I rebooted, and when it came back up, everything seemed to be working normally, and we proceeded with a series of flights that all behaved fine. Any idea where this sort of EKF divergence could come from?

Here’s a screenshot of the recorded pitch/roll/yaw during the incident.

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