I believe it was originally a one-time tribute for Robert Morris [1], but it has (partly due to user requests) been used a few subsequent times when people well-known in the hacker community have died.
[1] https://news.ycombinator.com/item?id=2712469. Besides being well-known in the hacker community generally, Morris also had a more local connection, as he was the father of Robert T. Morris, one of Y Combinator's cofounders.
