X-Files aside, there has to be a reason somewhere <lol> Since it's
happening in multiple programs the root cause has to be something that
is sending the 'paste' command. Have you a wireless keyboard and/or
mouse as radio interference could be giving false signals that the
computer is picking up as paste instructions.

Might be worth trying a different keyboard or mouse anyway to rule
that one out.

Once had a weird problem with a PC where the mouse pointer would fly
all over the screen and randomly click on icons as it did. This was
eventually tracked down to electrical noise from a failing PSU so
these seemingly random events can have all sorts of causes !!