I don't think reading other Discworld books would help in this case, it's a completely stand-alone occurance.

I think there are a few theories, but I think one of the following two is most likely:

1) The Glass Clock is designed to be completely accurate. To do this, it has to measure the 'Universal Tick' or, the shortest time it takes anything to happen. To measure something 100% accurately, it has to be static, unchanging. Therefore, the clock has to trap Time to measure it/her to the required level, and if time is trapped, then it can't flow.

2) The Glass Clock is designed to be completely accurate. However, time is flexible (as proven by the History Monks' manipulation of it). Therefore, the only way for the clock to be 100% accurate is to stop time completely, measure at that point and never let time flow again, allowing the clock to be always accurate.



I can't remember if either of those theories are explicitly stated in the book, I'm just extrapolating from my memory of it, so I could be wrong.