Why do volcanoes from tectonic plate boundaries ?

Well the divergent boundary, occurs most often at mid-oceanic ridges. Here, two plates move away from each other, forming a break, or rift, in Earth's crust. Magma from the mantle rises to fill the gap, creating new crust.

Hope it helps

Because the magma breaks through the crust at the places where volcanoes are found creating plant boundaries.