He resigned for a scandal about him having a mistress, who he forced to have an abortion.
The Daily Kos says he resigned due to staff abuse charges. As a left of center person I thought you would accept the Daily Kos as a source.
Another article
https://en.m.wikipedia.org/wiki/Tim_Murphy_(American_politician)
confirms this. Murphy announced he would not run in 2018 on 10/4 due to the abortion, marital, affair, and divorce mess. In the next 24 hours he was hit with staff abuse charges. On 10/5 the leadership decided those charges would have to be investigated. To avoid the distraction the leadership asked him to resign. He did so on 10/21.
Actually the abuse charges I think were worse than the other charges. I actually do not think the House would even have investigated the marital/divorce charges as their were no charges of abuse.
I do not think House ever investigates divorces or marital affairs were no abuse is charged. They probably would not be able to investigate anything else, if they did.