Tough to get oil into the fuel lines in any direction, but if I were to guess on how something like that could happen...
Fuel pump ruptured and oil pumped through the pulse line into the fuel. It got enough fuel/oil into the crank case to hydrolock it.
Now, I'm thinking this is either the 617 or 670 motor? Is there oil lines going down to the crank case? Typically to oil the rotary valve and crank bearing. I don't know what controls the flow, but maybe something happened there.
How it would undo itself is a puzzle.
2015 Renegade Backcountry 600
1999 Jag 340 Deluxe
1999 Z 370
1996 Touring E 380
1995 Summit 583
1994 Indy XTL SKS 600
1986 Tundra 250