Politics. A lot of people assume that union members truly have solidarity with one another. I mean in the sense of always favoring things other unions do. I understand why people think this -- because the organizations usually make a concerted effort to present a united front publicly. But it isn't actually the case. And the ILU thug even noted this in that interview -- when he talked about people in other sectors getting laid off because of supply-chain disruption and hating the Longshoremen. He was talking (at least in part) about other unions.
In my experience (and I have a pretty good amount of it in this area) union members are no different than anybody else. They are, first and foremost, concerned about their own well-being. They'll support other unions, but only to the extent it doesn't cost them anything to do so.
Timing If so, they're naive. As you rightly point out, the timing of this is not an accident. The shippers pretty obviously can't agree to swear off automation -- not in contractual sense, anyway. As such, the ILU seems far more determined to cause problems than to reach a settlement. They know they're demanding terms that cannot plausibly be met.
If Biden lets this continue to play out, he (and, by extension, Harris) will start getting the blame for not intervening in it. And I think he should be more worried about the votes at risk there than the votes at risk by using the tool at his disposal.
Principle OK, maybe. But I generally take it as a given that nobody who has been successful in politics does much of anything out of principle. And, frankly, that may apply more to Biden than most.