This is something I am seeing often.

I open CSS and get a list of available servers in Steam, set the filters to only display servers with openings, and sorted by player number in descending order.

I try to join servers that are suppose to have players, but when I attempt to join -- a pop-up appears saying no players are on the server.

Does anyone know what is causing this?

People dropped from the server, it crashed or you need to refresh your server list:)