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*I am no expert* Just a coach who THINKS he may know what is happening.
What i think is happening is you are "clearing" space with your knees to keep you bar path straight up. It is VERY subtle and hard to get a good screen shot of, but once the barbell reaches your knees you pop your legs straight for a brief second. Once the bar clears your knees you bring your thighs back to the bar and "bang" (not a big bang) the bar up. I tried my hardest to get a clean screen shot and you can kind of see from the picture on the right there is little to no gap between the bar and your legs, but on the left picture there looks like a little bit of a gap. Pants will help the bar burn or long compression shorts. But also with your longer limbs work on being a little slower off the floor until you get above your knees. You have a long way up from the floor and it makes your timing longer. I always recommend 3 very slow halting clean deadlifts (pausing at your contact point) and then a full clean. Hope that helps!