On a live gig, with a rotating hard drive, almost certainly vibrations, either through the floor or through the air.
I’ve had exactly the same problem when recording with a Pre Sonus desk – Acoustic set while people were eating was fine, but once the dancing started & the volume went up it was no good at all.
Solution is to use a solid state drive – I now use SanDisk Extreme USB 3 (I have both 32 & 64 gig versions) and have never had a problem.