Does Minecraft cause computers modems to go on the fritz?

How could a game affect your modems hardware? That's just not possible so the answer is no.
the modem is just a piece of hardware to get an internet conenction from the provider into your home then convert it from one type of conenction to an ethernet port, its like an adapter that changes your plug in from 2 to 4 outlets

No. Modems don't care about what traffic they send or receive. Their job is to just pass it along the best they can.

If you modem goes out it's because it was bad, not because of anything you were playing.