Any idea which Callistemon you have? Maybe drought could be the problem if it's a native of a very wet part of Australia, though they are usually very drought hardy even then. Sometimes a lot of rain after drought causes a problem.
What about the neighbours? Have you looked around the area to see if there are other callistemons of the same kind, and how they're doing?
The idea that pruning has harmed it don't sound likely. I wouldn't expect a problem from pruning a healthy tree unless it was pruned back very hard, and then it would be more likely to have died in quite a short time, rather than fade away over three years.
Has there been root disturbance? Building done in the vicinity? Change in the mulching regime? (Not everything loves mulch.)
Have you fertilised? If so it could have caused problems.
Trish