If you've recently seen Rainbow Six Siege players moving in peculiar ways, they may well be "crouch and lean spamming"—basically, mashing those commands to make their characters jerk in unnatural ways, making them difficult to hit. Ubisoft is aware that the problem is on the rise and is "actively working on how we want to approach [it] and planning our next steps", it said in a Reddit post.
You can see an example of the trick in this thread (credit Reddit user yap_panda)—notice how the other player waggles as they move out of cover, making it harder to aim at their head. In this case it doesn't work, but you get the idea. You can see a clearer example of crouch spamming in the clip below, uploaded by Reddit user Flash0509. Make sure you wait for the kill cam, where you can see just how quickly the opponent is crouching and uncrouching.
Ubisoft said it is "currently prototyping a few systems to address" the problem. I can see why it's a difficult fix: if a player is using macros to artificially crouch and lean faster than normal (as I suspect in the clip above), it might be easy to catch them, but otherwise, you could argue they're just using the mechanics available to them.
In the past, I've heard players suggest a stamina mechanic that limits the amount of crouching or leaning you can do, which seems like it could work if implemented correctly. Let's see what Ubisoft comes up with.
In case you missed it in the week, it's worth catching up on Morgan's in-depth piece about a Siege caster who is speaking out against toxicity in the game.