Games : What triggers sieges rather than assaults?

Sieges are the most troublesome invasion that my colony faces. The mortars keep setting fires and destroying walls, sending colonists outside to repair, who then get injured by mortar fire. It requires a great deal of micromanaging to minimize the damages.

Attacking the siege is dangerous; without the aid of my defenses, all I have is colonists taking fire and frequently dying or losing limbs. Each siege can be very costly.

My question is, what triggers an event to be a siege rather than a standard attack? Is it a wealth threshold, is it the way you have built your colony, research level, or is it completely random?

From my own observations, they seem to only happen once I'm settled in, which suggests wealth - but I've also built up my defenses and walls at this point.