Shaking legs when squatting and over use....why?

Im doing a form of Taichi that gives your legs a damn good workout and my legs shaking is the usual result.
We will hold Horse stance (squat sorta) for a few minutes and also do exercises with feet sholder width apart and your legs fully flexion(down) and then extension half way up and then back down again, repeated for 5 to 10 minutes....fun. Also hold each Taichi stance for 5 minutes with usually 75% plus of your weight in the rear leg and both legs in a flexion position.
The problem is that I find that my legs start twitching and then shaking up and down after a few minutes of any exercise. they shake more on the extension of the leg rather then the flexion. Sometimes they shake so much its nearly straightening my legs out. Im not talking a gentle twitch here...they shake all over.
What im asking you peoples is why does this happen? Also can it be stopped?
Im a fit 36 yo male with reasonable leg strength and fitness. 5'11'' 75kg.
No disorders (apart from once voting One Nation) of any kind.
Ive been training Taichi for 3 months now and the leg shaking is not getting any better.
Thoughts?