You likely have a burner that is blocked. Blockages can occur from corrosion build up or from insects building nests or webs. When the burner is blocked, the gas cannot fill the burner and builds up behind the front panel. Once it ignites, the flame is forced back out towards the front panel, resulting in excessive heat which could melt the knobs and bezels. Depending on the location of the blocked burner, the flame may melt the ignition box and any wires around it, as well. You will need to remove the burners from the grill to make sure they are free of debris.