Use your head, stupid!
All that the union bosses are concerned about is keeping the huge gravy train of campaign donations coming to Democrats, and that's why they find it so hard to follow the lead of how their members are supporting Trump overwhelmingly!
But remaining on the fence gives the union the opportunity to suck even more bucks from the DNC with implied blackmail. The DNC can either pay up, or suffer the reality even further if the Teamsters later chooses to endorse Trump.