How does the what community are remixing projects get there?

Ok fixed question abit so see again

People remix them a lot in a short period of time.

The exact algorithm is unknown to anyone outside the ST (and I'm willing to bet that a good chunk of ST members likely also don't know, as it doesn't affect their jobs directly- it's probably only those involved with the development of Scratch) to prevent people from intentionally manipulating site statistics to get on the front page, so that's all I can give you