It's because sorting comments by "hot" prioritizes new comments more than old comments even taking into account votes. So a 3d old comment with 50 votes might appear below a 2h old comment with 5 votes. Unlike Reddit which just pushes the first comments to the top and anything new will drown in the sea of comments and never surface or be seen.
That's not a memory leak though. That's just hording memory. Leaked memory is inaccessible.