The usual answer is that usually that the riders body position isn't quite right. Most riders that first learn to hang off in an attempt to knee down have a tendency to a) lean off the bike to far or they rotate around the tank. When you do the latter, it actually turns your knee downwards and into the bike rather down at the floor. I'd put reasonable money on the former being the cause myself, though without seeing, its hard to really tell.
Bullet
Honestly it just felt more comfortable for me to hang off 'tighter" to the bike in that turn because of its speed. I am not sure if that is the proper technique or not though, it just felt best for me in that particular turn to keep everything tucked in.
do you normally knee down, but didn't here? I'd buy that by the way, not everyone does, or needs to.
The more important to knee down is really what it's used for, apart from looking absolutely fab in those pictures on your desktop or Wall to impress all your non biking mates about how cool you are!
Bullet
I had my knee down consistently all throughout the day, just never in that turn. Was actually quite proud of myself, got to where i was using it as a gauge in the turns and would expect it to touch down at the same spot each lap. (it was my 1st track day )