@Hansern (I think), 'scissor' challenges are so frequent it would be hard to police them as a 100% can't do. Most come about due to the angle of approach, speed of the two players and the way the tackling leg moves, the following leg has to follow. I'm completely guessing but I would airily guess that the slow re-appearance of this is due to the players, both defenders and attackers, getting faster and faster.
In my day scissor challenges were done by fat bellied defenders as a means of stopping me making them run (and probably vomit!)