I'm thinking that Shero's medium to long term vision of where the Devs go next stopped aligning with what his ownership group wants. That's a guess, but their upper management was out doing press before the season, not Shero ... and they were talking a lot like they expected to finally become relevant in Northern New Jersey ... which means winning. That's why they spent the money on Subban; his community engagement being as important to the front office as whatever he's got left to offer on the ice. Probably more so, in fact.
Shy of any more details, I'd guess that Shero was gearing up to continue to build through youth and picks while ownership is tired of waiting for all that to pay off.