Leakage was probably their catch all term for ingress...
It's possible a grounding problem with some of the connected electronics which is using the router as a path to the coax ground and is inducing noise on to the coax.
It's also possible if the router is a wireless model, that it's RF transmitter is faulty and causing RF interference directly with the nearby modem or coax.
Either can cause enough interference to enter the cable system to impact OTHER customers.--
If it's important, back it up... twice. Even 99.999% availability isn't enough sometimes.