In a word: downvotes. Both by Martijn and on his posts. Downvoting answers costs 1 rep, and one of your posts being downvoted costs 2 rep. However, voting is secret, so the reputation lost for downvoting answers is only shown to you, not to other people viewing your profile.
If we look at Martijn's activity page, we see that he has cast 8 votes this month,...
At the moment Gordon Linoff, the #2 gains about 11000 quarterly reputation over Jon. With a difference about 121.000 it will take him 11 more quarters to overtake him. Meaning @Kaz's guestimate was about four quarters or one year too optimistic. At the moment it looks like it will take until 2024 until Jon is no longer #1.