What can a (startup) employee do to prevent these sort of situations from occurring after a liquidity event? In particular, what sort of terms should be in your hiring agreement -- eg. double trigger acceleration?
Check thefunded.com for the VCs (and ask around as well) and do your best to work for good people who care about their employees and reputation.
Not that that's a perfect solution, but it should at least make these more blatant acts a bit easier to pursue in court.