Shanahan was just following orders, speaking for the multi-billion dollar co-owners of the Leafs, who were proactively protecting their brand. It's all business, all the time, with Rogers and BCE.
With all his considerable strengths as a person and GM, Dubas was never the best negotiator and I think he mistakenly took on the meanest SOBs in town in trying to renegotiate at the last minute, after showing weakness* at the end of the season.
*weakness=admitting the job took a huge toll on his family and made him ambivalent as to whether this was the right job for him, long term. Been there, done that...