[OJFord]

ISP sees a new MAC address; it's perhaps quite likely that if you don't have a static IP it'd change on router replacement.

[sigmoid10]

Could also just be plain bad luck. Dynamic IPs are given out based on availability. So you could stick with one for months if you keep your connection active and then just happen to lose it during one of the moments you unplugged the router. To google this would seem like a static IP that suddenly changed.

[0des]

nope, this is a tried and true method i use to change my ip on non static service. I change the router mac address, flip the modem, and voila new IP

[sigmoid10]

Not necessarily. The fewer IPs in your ISP's dynamic pool, the bigger the chance that you will get the same one. In the extreme case when there's just one free address (I have seen this happen with some really shitty ISP) you'll 100% get the same one back.