What is/are the cause(s) of share rejection?
They are either invalid (bad hash of good data) or stale (good hash of bad data)
When a block is found any share you are working on or submitting is no longer worth anything. They are stale. If you push GPU too hard it will make mathematical errors producing invalid hashes.
In Mining > Pools I see occasional mention of rejection rates. I assume these are stale shares rather than hardware errors. Why would some pools (or mining software) be better than others in reducing stale shares?