Because of the build up of pressure. There is so much steam coming from such a compressed point, it’s coming out in force.
Now think of that same spot being closed, it only has one place to go but it can’t leave, so that pressure will build and build and then BOOM, it explodes.
In short, the answer is the pressure being released from a small point, and how that energy is released.
It isn't exactly known what causes the harmonic tremor or small earthquakes, but Hotovec-Ellis' belief is that they are caused by magma being forced through narrow channels in the Earth sticking to rock surfaces along the way. When it sticks, it takes a buildup of pressure to push it along further.
