I think alot of people do that just to make it known that they're skating switch.
When skating switch first started out, I made it a point to learn how to push switch too.
Everybody else just pushed with their normal foot when skating switch (mongo).
And as a result people would always get confused as to which way I skate.
I was the first person I remember doing that, but I'm sure there were others doing it at the same time.
As if anybody gives a shit. I'm sure yall don't.