A friend came up with this and I never heard of this problem. (Indesign CC-2014)
He makes a text frame with the text tool, and fills it with text by typing. Then he makes another frame, but after typing in one line, there appears a text thread link automatically.
This problem also appears in a new, empty document.
Does anyone know about this? This also happens on another device, same CC account.
Sounds like it might be Smart Text Reflow (see the prefs).
Found it! It was his (microsoft) keyboard. Num lock enters fill the whole text frame.
By default the Enter key on the numpad is the shortcut for a column break. Same thing happens on a Mac, if it has a numpad on the keyboard.