It seems like we've been in a constant flow of Avengers: Infinity War fan theories lately, but a recent idea about why Drax thinks being still makes him invisible is actually kind of funny.
Screen Rant shared a video on YouTube explaining the theory, and the ways that Drax came to his ridiculous conclusion.
According to a Reddit user theory, Drax's whole idea of being invisible as long as you're being still is a result of watching baby Groot (TOO CUTE).
During the end credits scene of Guardians of the Galaxy, baby Groot can be seen dancing behind Drax, but when he turns around, Groot freezes and it seems as though Drax doesn't even see him there.
The same concept happens again during the intro to Guardians 2, when Groot is dancing around as the guardians fight the octopus monster. As soon as Drax looks over at him, Groot freezes and it seems Drax doesn't notice him there at all.
The theory suggests that since Drax is an alien, maybe his vision detection is one that is based on movement, which would explain why he seemingly didn't, or couldn't, see Groot when he stood still.
So in Drax's mind, stillness = invisibility.
As Screen Rant addresses, it's a fair guess to assume Drax comes from a place so loud and eccentric that being quiet and still is admirable.
In the Infinity War scene the theory is based on, Drax seems to be present and eating his snack long before Starlord or Gamora notice him, so maybe