Maybe the agreement is "we will accelerate vesting of your unvested equity if you sign this new agreement"? If that's the case then it doesn't sound nearly so coercive to me.
Perhaps as an example of the blurred line; Pre-nup agreements sprung the day of the wedding, will not hold up in a US court with a competent lawyer challenging them.
You can try to call it 'economic' duress but any non-sociopath sees there are other factors at play.