Originally Posted by AdvRonski
The impact of a 500+ lb. bike on a sharp bump and a over-filled hydro-locked shock is the only way I can imagine blowing the reservoir off like that.
+1. It was either overfilled, or they buggered the reservoir cap threads when they took it apart. For the first disassembly, you need to use a heat gun to loosen the thread locker. If you just use brute force, the threads are likely to ball up. They could have also cross threaded on reassembly.
They would have had to take this apart to remove shims to lengthen shock travel. I would definitely demand that the shop fix your shock. 90% of the time, mechanical issues are related to the something that was recently worked on. I've rebuilt dozens of shocks and never heard of an issue like this.