We went on a ride on Sat. My gas was about half tank when we left. We did caught in the traffic a bit and the temperature was about 210~215. The gas gauge was at the last bar so I decided to put gas. When I open the gas cap, gas sprayed out from it and my friend who pulled up next to me waiting for me to put gas got more sprayed than me.

This never happened before and now I am scared to open the cap. Do you have any idea what caused this to happen?

 

You don't say which bike you were riding. So this is kind of generic...

Check the vent on the tank. Should be one, usually on the bottom.
You might have a gas over-flow, as well. My F3 does, so it has 2 ports.

Drain the tank. Check the hoses/nipples on the bottom of the tank
(besides the petcock). Usually, if there's two, it will be the smaller one.

With the gas-cap open, try blowing compressed air into the ports.
You should hear it exiting inside the tank. If not, try removing the blockage.
Compressed air or a piece of wire will usually do the job.
If you use wire, use copper or be sure the tank has aired out completely.
No sparks, static or otherwise!

Pull the petcock, while you're at it. Clean the filter-screen(s) on it
and then re-assemble. That's just good maintenance.

Afterwards, you'll want to rinse the tank with some kerosene.
Otherwise, whatever gunk was blocking the vent, will get into the fuel system.

Hope this helps, Ern

 

It's 600rr. dot net people said someone's tank exploded with same problem. This is beyond my capacity to check and fix. I will take it to the shop within next few days
Thanks