According to the article from the Cologne police that was linked above it was one female passenger who reported to the cabin crew that she had allegedly heard terrorist content in the conversations of the men.
It will be a fascinating to learn what happened in this "game of telephone" from panicky passenger via flight attendant into the cockpit. A rucksack which went through security control, a book cover with something related to "terrorist content" and three guys chatting about something related to "terrorist content" – these are not indicators for a terrorist attack. Quite the opposite. Neither would be present in case of an attack.
But somebody has to make the call of reason. If neither fight attendants nor pilots are trained and authorised to do it, and the information changes going from one mouth into the ear of another person, and with pervasive fear everywhere, we are bound to have these incidents.
Interesting though that they seem to happen mostly to flights coming from and/or going to US and UK.