From MacRumors:

A growing number of users on the Apple Support Communities, MacRumors discussion forums and social media have reported issues with the iPad Pro becoming unresponsive after being charged for a lengthy amount of time, forcing affected users to perform a hard restart to turn the device back on.

Count me among them. This happened to me twice on Saturday. Hopefully whatever the issue is can be fixed with a software update.

Update: Installing the 9.2 beta (currently beta 3) seems to have fixed this for me–at least for now.