The lightning port can essentially loosen over time, meaning the “snap” of the cable grips less and the plug can become susceptible to falling out. This has happened to me, even as someone who takes good care of his phone.
Even on very old very carelessly used phones I have not seen this. Every single time the lightning cable does not snap into place, it's some packed lint that is the problem. It can be removed with some care and it works fine after that.