I've never heard of anyone giving up like this without health, marital, or legal issues.
I wonder how much of Chaffwtz's decision is based on upcoming legal issues? Something big might've happened in the Chaffetz household? Or perhaps someone tipped him off that trump is going to be prosecuted hard and that Chaffetz's unwillingness to investigate anything trump related could possibly indict himself?
Certainly he'd still be a shoe in for reelection. To the best of my knowledge, this move doesn't make sense politically for himself or for the GOP. If the GOP wanted him out, they'd be hard pressed to find a better candidate in his district. Hughes and Bramble are probably the best known. Hughes is speaker of the house and will most likely go for governor. And Bramble is probably too old to make a House run.
If he does give up his position, I believe the governor will appoint someone in his stead until 2018's election. Which could be competitive if the republican candidate isn't well known or popular and the democrat's candidate continues to raise big money. She had raised more money than even Chaffetz!