Normally if it is footballing reasons - lost the dressing room, buying wrong type of player, the whole coaching team is sacked.
Shakespeare and Walsh are still there, and they have not resigned.
That suggests to me that this is entirely personal between Pearson and the Board.
Maybe he tried to defend his son's actions.
Despite what we read about the sleazier sides of Thailand the Srivaddhanaprabha family are immensely proud of their country and deeply religious. The sextape scandal must have caused them huge embarrassment.
Nigel does have a history of upsetting people. Maybe they asked him to change something, whatever, he obviously spat his dummy out once too often.
(My own personal conspiracy theory is that Cambiasso said yes to another year, but only if Pearson goes! No-brainer!)