I knew when I first starting reading this that the bike had recently been in a shop and that you weren't the mechanic. Upon further reading- confirmed.
Based on what
@TURBO200R4 said, I'll bet they did the first round of tightening and got sidetracked. Then, over time the nuts continued loosening up. If so, I'd think you might have felt some wheel wobble before it failed, or at least weird drifting in corners. No?
If they over torqued them that much, holy crap Batman!
As
@Kevin frazier said, best to do your own work, whenever possible. I think many riders don't because they think they'll mess something up, well....
I flew to Dallas years back to get a Triumph Trophy and got a flat in Arkansas going home. Got a new tire and the mechanic over-tightened the front axle bolt so much that the internal axle spacer was crushed and the bearings went out by the time I got home and figured it out. They paid me for those parts and your (hopefully ex) shop should too. Demand it!
For me, that was a big factor in my starting to do my own work. I don't have half the knowledge of many on this site, but I know how to read a cook book and just treat the service manual the same way. Most of it really is not, um, Rocket science.
Call corporate, if necessary, 'cause if the shop isn't at fault Triumph would be admitting that those bolts are capable of breaking under load, which is ridiculous/impossible.
I'm really glad that you were able to walk away from this and that mechanic is very lucky your family isn't suing him and the dealership right now.....