Definitely loose connection. Have to disagree about not soldering though. I soldered all electrical connections for 20+ years for house, commercial. and industrial wiring and had zero issues for circuits up to 440v 40 amps. Only stopped doing it when Code was revised. Then, surprise! started having some connection issues.

A loose connection will heat the wire and the solder can melt, granted, but stranded wire will burn up as well when connection isn't tight.