This is usually do to the height of the jump causing the leg to move because it is almost impossible to keep a completely quite leg over say a 1.50 M jump . However some riders do it intentionally to stay out of the horses way while in the air but this is all preference and perspective on riding.
Most riders grip with their knees, at least in my opinion.
I think it's them gripping with their knees or at least that's what it looks like. Most of the top riders that do the best normally don't grip with their knees and they have a strong lower leg and you can really tell the difference but that's just me opinion
Yes. Sadly a good bit of our upper riders do not focus on a stable and secure lower leg. I understand some horses have extremely difficult jumps to ride, but I know personally my trainer would never let me show at a height that I have not mastered with a secure and successful seat/ leg.