Possibly corrupted firmware. Although who knows how because your usage was as normal as can be. The MPK's USB causing your audio to garble is really odd.
Do you have a USB interface? That is the only way I could see the two (the distortion) being related as they would both be using the internal USB host stack/buss. If there is no USB audio interface involved that's one of the oddest things I've heard of.
To try to re-flash the firmware you'll need the Vyzex editor app. And here's the firmware file. (I don't recall the exact steps but the Vyzex app should have a built in pdf manual)
https://www.dropbox.com/s/h527yt3z4btwc ... e_v103.zip If you do have a USB audio interface I would unplug it while you run the Vyzex/firmware update.
If you encounter any issues trying to re-flash the firmware I suppose you could use a standard MIDI cable, but the MPK will need a DC adapter. ( Input 120V AC 60hz output 9V 500mA are the specs for a suitable plug)
I only ever had to do the firmware on an MPD and was just an update I had no corruption issue. Bit it did work fine. I hope my guess is right and this info is of some help.
Also if you haven't try a different USB cable. It's remotely possible your USB cable died right at that moment.
http://www.vyzor.com/support/index.php? ... c286f6f4ab