If the GM not making him a priority is enough to get him to want to leave, he never really wanted to stay.
If the team and GM didn’t value him, they wouldn’t have offered him $13M+ to stay here.
Priority wasn't the reason on its own. It was alleged from years before that there had been a grievance that Marner was not treated the same as Matthews & Nylander. The failure to declare him a priority like Treliving had for the other two would add fuel to that alleged grievance. It is a fact that Treliving declared Matthews & Nylander a priority and did not do so for Marner.
But beyond the lack of declaring Marner a priority (which members of the media noticed), the disclosure by the GM that he was exploring other options beyond re-signing Marner fanned the flames of media rumours. It put more media heat on the player for the playoff result because the team wasn't committed to keeping him around like they had Matthews and Nylander. By exclusion, he started to take a bunch of the blame in the media. As those rumours got way out of hand, the GM didn't exactly knock himself out trying to tamp them down or to defend the player. He gave it some lip service but he was quite limited in his efforts.
With both Marner's and his wife's family living here, that would be a little more to handle than whatever 'normal' might be. "95% chance he's gone by September!!!" as one member of the media reported, would have both families asking "what is going on?"
A kicker was that it allegedly really got out of hand. Metropolitan Toronto Police were verbally reported by TSN as guarding his house. I think it was Dreger and then I heard Jeff O'Neill loosely refer to it. I heard them with my own ears. It seemed to slip out and I don't think they wanted it out.
His wife went through pregnancy and then could be going through post partum depression (as many do). Crap like that really doesn't help. It was going too far. When police have to guard your house, maybe it is time to move on ...
You hear it all the time: it is a team sport. You're supposed to get each others back. Marner, their player, was a media piñata. Treliving gave it some token lip service "he's a star". But not the sort of forceful effort to shut the media circus down. He won't get off without some scrutiny. The MLSE board will want to hear how he lost one of their stars for close to nothing.
Dismissing this as merely hurt feelings over failing to name Marner a priority falls way short of what really transpired. It probably contributed to the cascade of what followed but it was far from the sole issue.