Why and how do Volcanoes erupt after being dormant for a long period of time?

Because the pressure builds up and eventually needs to be released.

Explanation:

Whether a volcano is actually dormant or not, is not actually predictable.

Volcanoes erupt when molten rock called magma rises to the surface. Magma is formed when the earth's mantle melts.

Melting may happen where tectonic plates are pulling apart or where one plate is pushed down under another.

Explanation:

I hope this helps you;)