The reason is rather obvious, IMO. Kasparov assumed, with good reason, that a computer is not likely to make tactical mistake. Especially in an end game the computer can see x moves deep perfectly. Computers historically are far weaker strategically -- doing things like creating good pawn structures.
So when Kasparov saw a move that appeared to be a tactical flaw his first belief wasn't, "Oh, must be a bug in the program, that was a random move." He thought, "There's an attack vector here I'm not familiar with."
It would be like sitting with John Carmack in a coding session and suddenly he deletes a function that seems like it would be useful to you. Your first thought would be, "Oh, John must have realized this function isn't useful. He's smarter than me. I just don't know the whole story." You likely won't think, "Must be a bug in John's brain that on occassion he will randomly delete a function."
Please be more clear with what you're saying.
Now, it might about the game if complete randomness is a valid strategy in Go.
I'm not talking about obviously bad moves.
I'm not trying to imply that Kasparov is a bad player either.
But why did he conclude the computer has seen a very complicated sequence that Kasparov himself can't even begin to see?
It's because of the nature of the game.
Remember, we're talking about a completely random move here ..