1624 emptied its ink bags all over floor last night

How old is the cap top on your printer?

 

Replace the cap at least once a year. This is only a guess but what could have happened in your case is the cap had swelled and contacted the print head surface causing the ink to be wicked from the nozzles and created a siphon effect which caused the ink to flow to the waste ink bottle.

 

No. I think your cap in good condition. But your maintenance station had a rest of ink in pump tubes. These ink goes to waste tank and created siphon effect.
Because If air leaking was between cap and printhead it can't cause ink flow from ink bottle to waste tank.
For prevent it in future you have to use Tubing Flush procedure in CR Maintence menu before switch off the printer.

 

How old is you printer? What type ink you using? I've 2 year old 1624x and I have same issue about month ago. Mutoh official said that I must turn of printer overnight. Issue was because ink valves don't close during cleaning process. Another Mutoh said that I can't setup waiting time cleaning.

What you think about that?

 

We had this happen on a Roland when a damper ruptured... but surely not all could go at once.

 

Check your lines first. Years ago my brand new Roland had the lines loosen from the cartridge and damper ends. Barfed ink out both ends of the printer.
Upside it was only the black and cyan.

 

That's not 100% correct. You should be putting the machine into sleep mode at night but not turning it off all the way. The valves don't really have anything to do with it. Mutoh VlaueJets didn't even have valves until the latest models and they worked fine for over a decade! The machine should always be set to clean when it's not being used.