While it’s always possible for a fluke to happen, odds are this is an error on their part. If they did not replace the tensioners and water pump they have made an error.
If the cause of this problem was a tensioner bolt coming loose, etc. then this would also be an error; either due to not tightening something properly or not examining threads, etc. before reassembling something.
The big problem you have is not being mechanically minded and not being able to sort out any BS you may be given if they go into CYA mode. (cover your axx)
I do not think you should authorize any repair at this point until the exact cause is pinpointed but it sounds like you have already done this. If so, this means any error on their part could now be covered up.
You could ask the shop to respond on this forum and provide some details. It should not be hard to separate the wheat from the chaff so to speak.