I'll toss in two more theories:

One, he lost track of the optics of vetoing 3 bills with high visibility and ideological importance to the American left: drug prices, drug decriminalization, and racial discrimination.

Two, the American left has low tolerance for ideological impurity - people are upset that he's not a progressive like Bernie Sanders nor a firebrand like AOC. However, the lack of those two qualities appeals to the middle, it might peel away some disillusioned Republicans. As another commenter said, "pick your battles." Newsom seems to be doing just that.

I'm not saying either of these is more likely than a coordinated effort to discredit him, just that they are other possibilities.