Why is RPN a head trip? Or rather... I don't think it's as much a head trip as prefix or infix notation. You've got your data. You've got your operation. Sometimes the data is on a stack. Sometimes it's on a heap. Just a little different way of specifying which is where.
Also... +1 on the "you've got to write it to understand it." And in the 80s, the documentation wasn't super. Leo Brodie's book was great to get you started, but understanding things like ' (tick) and how to program in "idiomatic forth" was a challenge. So I would add, "not only do you have to code FORTH to understand FORTH, you also have to rip someone elses' FORTH program apart to understand the more advanced bits." -- I could be wrong about that today, it's been a while since I did a survey of FORTH documentation.
> Why is RPN a head trip?
It isn't how we teach math in the two countries I've lived in. Lisp is just as weird. Many people see a mathematical expression and panic. A level up from them, people see a mathematical expression in some source code and expect it to respect the symbol precedence that they were taught in grade school -- which they can "understand" without knowing how the language parses and abstracts all that away. And maybe their understanding is flawed but they can survive as programmers for decades without ever going deeper. Lisps, FORTHs, etc., don't allow you to proceed without understanding.