European airports are often picky about cables.
They want anything remotely electronic separated and spread out.
No iPads left inside bags. No chargers or AC adapters left inside bags.
I had this happen even in Portland, Oregon. I left an ordinary laptop charger inside the bag, the cord wrapped around it, and it got their attention.
I've also been asked to put a mobile phone into its own bin.
I don't think it was "suspicious behaivor" and I wish that the Fraport agent wasn't shaming about it. The more polite response would have been like the friendly woman in Portland, "These sometimes look strange on the x-ray, and it's hard to figure them out; that's all."