The Dutchman has been criticised for walking out on Southampton.
A rpeort claims Koeman wasn't universally popular at his former club
The Dutchman's exit has left a bitter taste in the mouths of some on the south coast - notably his former goalkeeper Artur Boruc, who hinted on Twitter this week that the decision was financially motivated.
However, according to Ashton, that's not entirely the case.
Ashton writes that while Koeman will net an extra £1 million per year by moving to Goodison Park, a fall-out with a key member of Saints' staff was also a factor in his departure.
Koeman, along with his brother Erwin, are said to have been 'agitating' with head of football development Les Reed for some time, with a disagreement over the club's academy players 'chief among the issues'.
Reed (L) with England manager Roy Hodgson
Ashton alleges that while Reed wanted more academy players training with Koeman's first team, the 53-year-old was unimpressed by the quality of some - and at one stage banned them completely.
A Southampton statement, issued in the wake of Koeman's exit, which failed to mention the man who recorded consecutive record finishes in the Premier League, is perhaps evidence of Saints' feelings towards their now ex-boss.
Though, despite the club's stance, Ashton writes that contrary to claims, Koeman was 'extremely popular' with most of his senior players.
The Koeman brothers