beginner casting problem

I've been learning to flycast for about a week now and am having
success at short distances, but after several minutes of casting, my
tapered, 10 lb. 8 ft leader becomes severely twisted and knotted.
After straightening it out, the tapered end is permanently crimped. I
know this can't be right and must be severly weakening the leader
material.

Can anyone point to a likely flaw in my casting technique that would
cause this?

thanks in advance.