Why does tomato soup "explode" when boiled? When you boil tomato soup it very easily "explodes" when reaching the boiling point. Why is that?

A related question has been answered here before: Why do some solutions eventually stop boiling and start &quot;popping&quot; as they get more concentrated?
However, the answer to that question does not fully answer the current question since it does not address what properties in a solution causes superheating/bumping. That answer describes what we see but does not describe WHY it happens (except for the words "heated too rapidly", but many solutions can be heated with the same speed as tomato soup without "exploding").

For example, to my knowledge tomato soup only suffers the risk of "exploding" THE FIRST TIME it reaches the boiling point. Therefore I have thought the reason for the explosion could be that it contains gases that leave the solution violently. (I have read this is the reason that milk also may "explode" at the boiling point.)

Friends if mine suggest that the reason for the explosive boiling is the viscosity of tomato soup. But this answer would not explain why tomato soup only risks "exploding" the FIRST time it boils. (Without extensive experience I would also say that NOT ALL viscous solutions boil explosively.)

Tha tomatoes are naturally peeled so their elastic peel/membrane does not come into account. 

The previous question also relates to gradual concentration which is not the issue of the current question. The current question relates to reaching the boiling point.

So this question is not answered. To rephrase it slightly:
What properties in tomato soup causes it to boil "explosively"?
I think it's probably a combination of things you mention. First, tomato soup is a suspension of solids (tomato tissue) in water. So it's the water that's boiling. That water is turning into steam, which bubbles out of the water. The solid suspension likely traps some of that steam until the pressure builds and you get a big explosive bubble.

I happen to cook, and make a fair bit of tomato sauce, which is very similar to tomato soup. I disagree with your statement that tomato soup explodes only once upon boiling. I see bubbles popping quite continuously, and you have to stir often to prevent this. Which happens to also be the way you prevent bumping in laboratory conditions. Cooking is probably the most comparable chemistry application most people are exposed to.