The nozzle definitly hit the part and knocled the bed off. Usually the belt would have slipped before enough force was applied to actually knock the bed off the mounts though. You can see what looks to me like the impact zone on the front edge of that part, though it looks like it hit with much more than the tip of the nozzle. It looks to me almost like the Z axis reversed (which I have heard of once before due to overheating due to fan failure or improperly vented enclosures on a Taz 3 and never since) and started driving downwards into the part. Then it hit one or more times and eventually knocked the entire assembly off. The position the Z axis is sitting at in the picture is lower than I would expect to see at that stage of the print. It’s also possible you lost a bed washer under where the bed is sitting, the bed flopped up and hit that way
The damage doesn’t look catestrophic. You can apply a new PEI sheet sourced from Lulzbot or IT works. The bed wire harness and the nozzle heat brake tube would be my biggest worries. You may also be looking at replacing the rods if you bent any of them. Possibly the bed mount plate too if the corner bolts stripped out on any of the corners.
As to what caused it, the biggest factor is printing unattended. Please be aware that without the thermal runaway protection firmware on the Taz 6, you probably would have burned down your office. You should never leave a major heat source like that unattended. Period.
All the Taz 6’s are new enough they should be under warrany, but you would need to check with Lulzbot to see if they will cover you in this case. It’s possible this was due to a control box fan issue or something along those lines, but its also possible thatit was caused by a printer in a small room operating in a building where the air conditioning is shut off at night and the thing overheated due to user error. They may want it back if nothing else just to investigate the failure and see what actually went wrong.