Roblox recently introduced a security check to combat the botting issue. When you join a game you have to solve a “challenge” packet (you basically get a protectedstring that you have to execute and send back the params".
Recently Roblox changed the required time to solve this challenge from around 30secs to less than 10secs. If you have an antivirus or something that filters your network traffic and if you combine that with many opened programs that consumpt your bandwidth, you will start getting kicked often for this specific reason.
Afaik Roblox doesn’t provide you with the exact hackflag that you’ve triggered, maybe they should do that.