Why do (some) silicic bodies behave effusively instead behaving explosively as in common?
Explosive volcanic events happen mostly because of saturation in gases, and increase in pressure above the strength of the top of the magma chamber. As the magma decompresses or cools, H2O (mostly), CO2 (also important), and other gases such as H2S, SO2, HCl, etc, form bubbles and increase in volume. If the magma chamber is confined, there is also an increase in pressure.

An obvious corollary is that gases have to be present. If the magma is mostly dry and volatile-free, there are no volatile species to form bubbles and the magma cannot explode.

Another factor is the viscosity of the magma. The less viscous it is, the easier it is for the magma to flow (and not build up pressure) and also the easier it is for bubbles to escape the magma. Viscosity of magma depends on temperature (hotter = less viscous), composition (more alkalis = less viscous), crystal content (less crystals = less viscous).

Taken together, a hot alkali magma without crystals and no volatile species will flow, whereas a cold partly crystallised wet sub-alkali magma will probably explode, even if they have exactly the same SiO2 contents.