They are not really supposed to be going so close - it is against official advice anyway. The scientists are in planes and helicopters.
Lava moves very very slowly generally, so it cannot 'catch you' but it theoretically could spurt if you stand in the wrong place, or if it hits a wet spot there could be a steam explosion. I believe the main danger is from poisonous gases - I assume the people are being careful to stay upwind at least, but if the wind changed or stopped they could be seriously poisoned, especially because the fissure is in a valley.