If the liquid flow in the panels becomes static for any reason (e,g, pump off because tank has reached max temp) then, if the pump comes back on during a period of bright sunshine, the initial flow of thermal fluid could be coming off the panels at well in excess of 130degC. I think I've measured something around 150degC on the incoming pipe from the solar thermal panel to the hot water tank. Even with the increase in boiling point caused by the 50% propylene glycol, coupled with the system pressure of nearly 3 Bar, the system can get very gurgly, when that happens.
