This is speculation, but this doesn't have to be all that unreasonable:

Since OP had only 15% it doesn't seem likely that this was a "The Social Network" style trick to actively screw him out of his equity. More likely that it was really an internal round - no new outside investors could be found and the company was out of money. OP had left the company by this time and didn't value their shares enough to participate. Presumably the 15% had been vesting over 4 years, so a company that is out of money, can't raise more and had been at for >4 years, suggests to me that the 15% was really not worth much before the internal round.