It erupted because the bulge (which had been created of gasses etc) slid away in an earthquake. This exposed all the super heated gas/steam enriched rocks which made everything lose pressure. The volcano in reaction, exploded.
Why Did Mount St Helens Erupt?
Aicha is always so doggone thorough, did he say anything about the tectonic plates in the earths crust that are constantly pushing and pulling, above and beneath each other which creates tremendous pressure and heat which rises like steam and takes everything, in its way, with it. Deadly gases, molten rock, surface rock, etc...All go boom! This I realize is simplistic but I am not as smart as my friend Aicha, good call A. 
Because of the plates
There was a great volcano that measured 5.8 on the richter scale. The volcano was woken after 123 years of sleep and erupted.
A 5.1-magnitude earthquake triggered a landslide, which took off pressure that had been holding in magma.
I think it was on a Ring of Fire plate or something. I'm not an encyclopedia, sorry.
Because your dads dik got there
Wow. I am shocked by the rudeness of the young person who asked this question. Aicha was just trying to help and the young student tore her to shreds. Do your own homework kid and don't insult people who took time out of their days to help you.
Mount St. Helens is an active Stratovolcano which is located in Skamania County, Washingtom. Mount St. Helens erupts because it is located in the Cascade Range and is part of the Cascade Volcanic Range. It is a pacific ring of fire which has over 160 active volcanoes apart from Mount St. Helens. The last deadliest eruption of this mountain was in 1980.It has lava rock interlayered with ash, Pumice and other deposits. It also has layers of basalt and andesite through which several domes have erupted including the 1980 one.