First of all, sorry about your damage.
I can't see any other explanation other than either too long pto shaft that jammed the tractor, or the implement bounced up on that potential rock you mentioned creating a severe angle on the shaft putting high lateral stress on the tractor shaft.
I snapped off a lower hitch arm mounting point on my L4060 last year. It was cast into part of the transmission case (brake case) and set me back around $2000. This was 100% my fault... Mistakes happen.
If the dealer sold you both the tractor and the tiller one would assume that it was properly fitted. The tiller seemingly damaged the tractor during reasonable operation. I would push back a little harder.