They are looking for something to hang on to, brush, trees, etc. so a human is just another object to land on, so is your fishing rod. The mandibles are for clinging on to the opposite sex for mating. I've come to respect the Dobson fly, it means no harm but is equipped with serious pincers.
"Although not to the same extent as the larvae of mayflies and caddisflies, hellgrammites are intolerant of polluted waters and may have potential to be used as indicators of water quality." < Wikipedia >