That is corrosion due to flux residue from the soldered joints not being wiped from the outside or flushed from the inside.
There are several grades of flux and some are more aggressive than others - I use a very "soft" one for all tasks but make sure to clean both the pipe and the fitting, reserving the aggressive one for the very few joints that don't take.
Timeline is also about right - I had one go on me a few years ago. it weeped into a concrete floor for about 6 months until the evidence became visible.
Based on the evidence, there may be other joints being affected - check the ones that are visible. Note that some may have been cleaned on the outside as part of normal house cleaning, so find some under floor or behind cupboards etc to check.