Why Pourchaire had to leave Arrow McLaren in 'horrible' fashion

Theo Pourchaire did not see it coming. The Frenchman - on loan from Stake F1 (Sauber) - was preparing for the next IndyCar race at Laguna Seca, until he received a 'horrible' phone call. The communication from first his manager and only later his team Arrow McLaren was short and to the point: 'You are being parted with immediate effect and Nolan Siegel is the replacement'. Formula 2 champion is still furious. But just why was he cast aside by McLaren's IndyCar branch?