True that the company needs to follow regulations. But they could do "net exercise", or "sell to cover". Instead they choose "pay cash or forfeit" path.
I'm not convinced these events represent a significant enough number of shares to move the price, though. And regardless, many current and former employees will use this opportunity to dump more shares than just to cover withholding. I wouldn't be surprised if the number of shares sold normally at that time will dwarf the number sold to cover tax withholding.