To be clear, no-one knows why Trump vetoed in the first place (his first two excuses were clearly not particularly credible; one could believe them if one liked, of course, but it’s not unreasonable to be sceptical of them). So one is left with speculation. This seems plausible, though obviously no-one can say for sure.

It’s perfectly reasonable to speculate on reasons when someone does something weird, and risky (ultimately, he was overridden) for no obvious reason.