On some level it seems that Trump is employing this “emergency Powers” over reach - and the veto - as a sort of testing of the waters - process validation - desensitization - normalization to assess the usefulness , potential for success and over all utility of this option for executive action in the future.
He has clearly encroached on matters that are the Congresss’s to decide - and now he knows how many Republicans are willing to roll over and ignore the constitution and obediently do his bidding …
also - that “I am ordering the grounding of Boeing 737s” was not just an out of control ego (which it was in part) - it conditions the population to accept his unilateral orders as ‘acceptable’