There is probably some mechanic that does that, so it just got activated for weapon modding. I didn't just pull it out of my arse, if that's what you're thinking. I'm thinking crusherknack is probably that weapon, it's a secondary effect of the Smasher.
Ah if only we could get the ability to add recruitable troops and commander to non starting sites too. That would be the icing on the delicious patchcake.