The Greens threatened to topple government a few times and stood in the way of $10/day childcare and easier access to unions. Horgan didn't want to steer through a year of uncertainty with a shaky foundation, you really need to let this go that it was all a power grab.
Also, Horgan made a deal with Andrew Weaver's Green Party. Weaver has left the Greens and has gone on to endorse Horgan. That alone speaks volumes.
According to who? Horgan called the election four days after the Greens picked a new leader. Sorry but I didn't just fall off the turnip truck.