Did the Z axis actually move or did your end mill slide out of the collet? An upcut end mill exerts quite a lot of downward pull on the end mill. If the collet nut isn't tight enough, it will pull the end mill out and destroy your spoilboard. I've had that happen before (have the mark in my spoilboard to remind me constantly). Be sure your collet is tightened appropriately. If you have a spindle, it is nearly impossible to overtighten the collet nut.
ShopBot Details:
2013 PRS 96x60x12 (Centroid upgrade)
4hp Spindle
12" indexer
Aspire
Rhino
Fusion 360
Ferrari 360
Prusa MK3S+
Prusa XL multi-tool