Not sure why this is being downvoted. This is true.

Why would someone call their theory "trickle down"

Like waterfall/agile, trickle down economics is the name used by critics of supply side economics, not it's creators. It's use instantly reveals you are a critic, not knowing that isn't what it's actually called reveals ignorance on the subject of economics more generally.

Or as G.H.W. Bush put it, Voodoo economics.