Gyroscopic forces and simple resistance to tilting doesn't explain it:
http://www.losethetrainingwheels.org...px?Lev=2&ID=34
[the Zero Gyroscopic Bike] acts almost indistinguishable in handling as compared to a conventional bike.
Also read the wikipedia link posted. It's not a simple answer.
My single speed bike is an old road bike with a non-stock fork. It's impossible to ride no-handed for any distance. I didn't know why this was until I noticed that it has nearly zero trail. With even a few fingers on the bars, it's fine. I think I keep steering it underneath me without consciously noticing.
Track standing works more like a unicycle. It's a dynamic system of balance; you can't track stand with your brakes locked. You fall each way for a short period of time.