Mt. Rainier is part of the Cascade Volcanic Arc, which extends from Northern California north into Canada. These types of volcanoes are most known for their explosive eruption types. Mt. Rainier is no exception to this, it has produced major explosive eruptions in the past and will in the future. This type of eruption is generated through having a highly viscous supply of magma that does not allow for the readily release of volcanic gasses. The pressure builds and once the onset of eruption occurs, explosive decompression of the magma chamber can ensue.

it is an explosive volcano (erupts ash, rock, debris, and the like) and one of the most dangerous in the world

The mountain itself was put on the decade list for the most potential threat to people and property within a decade, however there have been no signs of the mountain re-awakening in the near future. CVO's website does do weekly updates every friday on the current conditions of the volcanoes in this volcanic arc.
Mt. Rainier can erupt explosively like any other stratovolcano, however it can also erupt passively. It simply depends on what is going on inside the volcano that is triggering the eruption and how much energy is stored in the magma chamber. As a side note Rainier doesnt necessarily need to erupt to cause significant damage. The mountain has the highest concentration of glaciers and a simple rise of summit temperature could be enough to trigger a lahar.

The Cascade Mountain range was formed by pressure from the collision of the Juan De Fuca plate and the North American plate. The Juan De Fuca plate is constantly being pushed into and under the North American plate which pushes up the North American plate where the Cascades are.

ocean-continent convergence.