A recent incident at a SmackDown taping led to WWE's new thigh/leg slap ban.
Per the Wrestling Observer's Dave Meltzer, Vince McMahon witnessed one of his wrestlers fairly obviously slapping their leg while throwing a kick on this SmackDown show, leading directly to his decision.
There is currently no word on how WWE plans on enforcing the ban, though fines will be handed out to anyone caught slapping the thigh.
Meltzer adds that signage reading "do not slap leg when kicking" has been put up near the Gorilla Position.
Slapping the thigh is common practice for wrestlers who want to make their kicks sound more impressive. Tune into any modern wrestling show in any promotion and you'll see it, though most performers do a good job of hiding what they're doing. The aforementioned SmackDown wrestler appears to be the exception.
Thigh slaps aren't always restricted to kicks, though: NXT's Johnny Gargano once did it while chopping Velveteen Dream.
As is often the case, Randy Orton has some fairly enlightening thoughts on the matter:-
This led to what looked like legitimate beef with NXT veteran Tommaso Ciampa, though there's no word on whether or not Orton ended up attending #legslap class.