For references, the rule book goes over the cast spell steps and the fact that all spells have to follow them. The spell itself says it generates the attack when it resolves, and that it is unavoidable. The rules again go over combat steps which all attacks generally follow, but the official rules and codex supplement breaks it down into more detail showing that attacks like explode only skip the damage barrier and counter strike steps (because you can't hit a spell back, and you weren't hit by a creature/conjuration) as well as the additional strikes step. It still goes through avoid strikes step.
The supplement also clarifies that the spell explode is the source of the attack, so no traits on the caster modify it at all. Finally the supplement clarifies that forcefield works against unavoidable attacks.
Sorry for lack of page numbers, I'm on a mobile device without access to physical materials, making it difficult to reference.