Hi, this is normal to happen. I've been managing accounts for 7 years and you are always going to find these "weird" things.
Basically what is happening is that an image which is in one campaign got more clicks at the very beginning so then Facebook rewarded this image (under that specific campaign) making it cheaper and problably increasing the frecuency of the ad shown per user which is more likely to be clicked.
On the other side, the other image on the other campaign lost the "competiton" so Facebook penalized it with higher CPCs and probably with lower frequency ad per user thus, lowering the changes to get more clicks.

Basically you are cannibalizing and competing with yourself. I would recommend testing new images in order to continue undertanding which type of visual works best for you in order to get better CTRs and conversion rates.