Why does my radiator attract wasps/yellowjackets?

I am noticing a strange phenomenon on my motorcycle radiator: when parked, nearby yellowjackets flock to it and seem to get quite aggressive. I’ve actually noticed one of them stinging the radiator!

I’ve also noticed this happens with parked cars. Wasps fly in and out through the grille and seem strangely attracted to the area surrounding the radiator.

It also happens when the engine is cold, so warmth is probably not the answer. My theory is that the dead insects on the radiator excrete some kind of chemical that angers wasps. Has anyone here noticed this phenomenon as well?

(Note that I live in the Netherlands and by "wasps" mean the smaller kind that is probably referred to as "yellowjackets" in the US)