It suddenly occurred to me that DoctorBass' explanation makes the motor oscillate a very tiny bit but that this resonates the belt like a tuning fork. (Reminds me of a Navy story involving an 8MW generator where the oil pump impeller had sheared, causing carbon brushes to break from oil pressure transients causing tiny oscillations.)
Reading this made me think of hitting the belt's resonant frequency and I suddenly had this image of my bike breaking to pieces due to a chain reaction caused by the belt vibration! Haha
It made me think of this old video of the Tacoma Narrows bridge...haha
Thankfully, it won't do that! "Resonating" in my manner of speaking is a fairly general phenomenon. (Another program I was involved in took six-axis accelerometer readings on bearings, axles, etc. and then we stared at FFT/wavelet breakdowns with some assistive software to identify upcoming breakdowns and wear.)
But yes, I've had that engineering story burned into my brain like the per-minute forensic logs of Chernobyl and (less catastrophically) Three-Mile Island.