I'm not sure what the criteria is for trending, but it isn't sales alone - Rimworld is #3, and there are plenty of AAA titles in the top ten. GTA5 has been in the top 10 of sales for months and months. Also, I think Rimworlds positions also includes everyone that owned it pre-Steam that is now redeeming their Steam codes.
http://store.steampowered.com/search/?filter=topsellers
No doubt that Rimworld is doing well, and I hope continues too.
http://store.steampowered.com/search/?filter=topsellers
No doubt that Rimworld is doing well, and I hope continues too.