Ok, my former answer was obviously wrong, so I will try again.
I don't remember the exact details of how What's hot works. But I took a fast glance.
Basically it's based on the number of favorite points / number of logs a cache has gotten the last 30 days. A minimum amount of logs during that period is required as well. The cutoff seems to be 10 logs.
We do not want to post the source code of it publically.