It might have to do with the fact these compound effect some of the same neuroreceptors in similar ways. Disclaimer: I'm not a chemist and have only very basics knowledge about chemistry.
I know the quote below isn't talking about aliens but I too had some "alieny" or higher power feeling to a lot of my trips.
"As you can see, psilocybin is quite close to DMT in its structure, and thus fits smoothly into the receptors. LSD is sufficiently different, which explains its more alien qualities."
