Meddling Mara.
Tbh, that's the reason I think Dabs/Schoen are still employed. They did what the owner wanted and it blew up. He had enough grace to not pin it all on them and fire them when it was clearly a disaster.
Like @GoBills808, that lucky season where the G-Men snuck in validated Mara in a way and put them in a really bad spot. I think the Giants have a brighter future today if they went 6-11 that year instead. It was a little bit different than the Bills situation where TT was actively quite bad at QB, had already taken a paycut, and was closer to the end of his career as a starter. The Bills were all aligned TT wasn't the guy, regardless of how 2017 ended. The Giants were not.