It is an induced pulse from another signal. Hanging a scope probe puts a very tiny but just enough capacitive load to eliminate the narrow pulse.
Probably caused by a poor connection on that signal somewhere on the board. If you can't find it - get a 15-20pF capacitor (similar value to your scope lead which fixes the issue) with a 100V rating, attach between pin 3 and ground. Yes, a kluge but for a one-off fix...