It's all very well speculating but the bottom line is we don't know what the charges are.
In my opinion it can't be a matter of gross misconduct or other serious charge because that would have been grounds for instant dismissal whilst still retaining the right to compensation from any club...
Not only "constructive dismissal" it could also well be a serious breach of the contract between the club and GP.
It is likely to have included clauses such as, "The manager, (Gus Poyet), will be solely responsible for selecting the team."