Why would a 15mm copper connection suddenly start leaking?

I've got a 90 degree elbow under my sink for the 15mm (all copper) from mains (with an isolating valve), I've got a T further up, with my dishwasher connected and a basic inline water filter to the tap (mixer).

Before I go and replace the connection, is there any reason why there might now be a random leak? It's just a drop, like a small bowl takes a couple of days to fill, but it was fitted two years ago and has just appeared this week. Not a plumber, so just curious if there could be anything else that might be causing it.