Twitter locks staff stock accounts in anticipation of deal
reuters.com
reuters.com
I think most US publicly listed firms use them, but usually only for senior executives; I don't think there is any reason in principle why a firm couldn't make them available to all employees.
If you work at say Goldman there's a window every quarter AFTER earnings that you are allowed to sell your stock. Otherwise no dice.
So no, not market manipulation, just regular business.