I am not sure; I suspect some stray voltage/current accelerating corrosion. However, this joint likely has something (gasket, seal, lubricant) isolating it from the corroded pipe.
The other options are alloys that are more sensitive to whatever is in the air 
Either am I.
There are tons of “theories” in the comments, but only one or two that seem plausible.
As this same set-up is common in my area, thought I would get a ‘heads up’ for if/when I run into the same issue… as ProPress becomes more and more popular… (aka. LPS)!
ProPress fittings have an O-ring inside.
My guess is the press fittings are brass and not copper and is creating galvanic corrosion.
Propress is a brand name and from what I understand the press fitting can be made from Copper, Brass, or stainless steel.
That’s a head scratcher. Maybe the plumber changed to a copper pipe from a different source that is a different and less reactive alloy.
With that being a factor, I wonder if it is an electrical issue, given the grounding system??
(Thinking I should extend this to the Electrical section, as our Sparky experts won’t typically visit the Plumbing threads).
There is metal to metal contact with propress fittings so I doubt the o-rings have an impact. I’d lean towards different alloys of copper. Its been said here many times that new copper pipe is not the same as the old stuff. I’ve been in houses where newer 20 yr old pipe and fittings had more corrosion and pin holes than the copper from the 1950s it was connected to.
I have a few theories:
1 - As other stated, poorly made copper and not as pure as it should be, hence the patina on it.
2 - I have seen copper corrode quickly when exposed to a sewer gas leak, but it turned it more of a black color. It may have been discoloration at some of the older pipe, and none at replaced pipe that had not been exposed.
3 - There was insulation removed on the clean copper, and none on the patina copper - but this is a stretch too.
Either way, makes me curious what the root cause was.