In the event this isn't a troll post, please state:
What fuses? The line fuse/circuit breaker at the panel? The fuses in the cabinet? What model are you talking about?
You could have a dead short across a station or tunk pair on the cross-connect block, you could have gotten a bit of cross-connect wire into the system by accident, some component failed - like a hard drive, tape drive, or even the power supply itself.
Usually fuses blow when there is too much load across them. A fast-acting fuse will respond quickly to an overload, and a slow-acting fuse fails over time.
Back in my CBX 8000 days I could blow a fuse just by simply accidentally shorting across a pair while cross connecting. The newer stuff seems more resistant to quick bursts like that, but I'm sure a sustained short would trip something out or blow the port on the board.