I also don't see that any mention of Mark's Dad before we first see him would lead a viewer to believe he was a pushover. Also find that such a strange remark.
As has been said above, the seeds are planted earlier on in the series, Mark references how his Dad controlled him and how generally resentful he is that he wasn't allowed to follow his dreams but instead sat under the thumb of an old miserable dismissive narrow minded cynic only to become a mediocre pen pusher convinced life is all about grinding through.
The guy who plays the Dad is really excellent in that episode. Grimly trying to cling on to the remaining petty aspects in his control as his shitty wan light fades gradually from the world. He is a total cunt and yet utterly believable. You could go outside and find someone just like that in half an hour.
