Two ways impeachment could happen:
- Trump becomes an albatross on the GOP, to the degree that they lose the House in 2018 despite their geographic advantage (the Democrats would basically have to win the House popular vote by 6-8 points in order to get a majority of seats, as per FiveThirtyEight). In this case, the Democrat-controlled House would be quite likely to initiate impeachment proceedings, both because Trump is a worse President than Pence would be, and because it would put GOP Senators in a serious bind.
- The GOP preemptively impeaches Trump, both to prevent a bad election cycle for them and because they would prefer Pence as President.
These aren't that unlikely over the next four years, though it won't happen in the first 100 days (barring some bigger bombshell or some real civic virtue from the GOP House leadership).