What would cause a PEX water line to suddenly develop a pinhole leak?

The red color suggests it's a hot water line, wich is exactly where these types of PEX failures ussually happen. If that repair was done about ten years ago, you might be dealing with a specific manufacturing defect. There was a period between roughly 2010 and 2015 where a major manufacturer produced PEX-A tubing that ended up being prone to **oxidative failure**. The pipe basically gets brittle from the inside out due to the combination of heat and chlorine in the city water, eventually leading to those tiny pinholes or hairline splits.

You shold check the printing on the side of the pipe if it's still legible. If it says Uponor or Wirsbo and has a date code from that era, you’re likely looking at a systemic issue rather than a one-off fluke. When that stuff fails, it doesn't just stop at one leak. It’s worth cutting out a larger section or at least inspecting the other plastic lines you mentioned. If they're all from the same batch, you might have more problems waiting to happen. Another common culprit is a 'slow' mechanical injury. If a drywall screw just barely grazed the pipe during the initial repair, it might hold for years before the constant expansion and contraction from the hot water finally causes it to let go. But since you described it as a pinhole and mentioned the age, my money is on the material itself failing due to the water chemistry.

Since you've already got the wall open and the insulation is soaked, I’d pull back enough sheetrock to see the nearest joints. If the pipe looks 'chalky' or has any white discoloration near the leak, that's a dead giveaway for chemical degradation. In that case, I'd definitely replace every bit of that specific plastic you can find with new copper or a modern PEX-B to avoid dealing with this again in six months. It's much cheaper to fix it now while the desk is moved and the tools are already out.

That's scary about the chlorine literally eating the pipe from the inside. I’ve got some blue PEX in my crawlspace that looks similar—does the color actualy change how the pipe reacts to the water, or is it just a coding thing? If it’s a chemical issue with the city water, does that mean the cold lines are safer since they aren't getting blasted with heat? I’m a total newbie at this and realy don't want to start ripping out walls if the cold side usually holds up better. It’s wild how a tiny pinhole can cause such a mess behind a desk.

To answer that question about cold lines, yeah, they usually last longer beacuse heat accelerates the chemical reaction with chlorine. But don't assume they're invincible. Another thing to check is how tight the bends are. If the installer kinked the pipe or forced a tight turn without a support bend, that spot is under constant stress. Over ten years, that stress combined with the hot water is a recipe for a blowout. I’ve also seen PEX fail early because it was left out in the sun at the supply house. UV rays kill this stuff fast, making it brittle before it even goes in the wall. Definitely check for any sharp kinks while you have the wall open.