He probably removed the bolts completely. And then a couple of trains had driven over that area of tracks. If each train pushed each track out about 1 cm in both directions then you might get those results. Extrapolating, after a few more trains spread the tracks a bit more you get the derailment.
Thankfully he didn’t do it in summer. The temperature is mild at the moment.
Continuous Welded Rail gets a life of it’s own during temperature extremes.