For anyone that's not familiar with the issues or didn't read the article:
In the past month there have been 5 hypoxia events in F-35As at Luke AFB, Arizona; the main training base for F-35As.
As such, they've grounded the jets just for this weekend, with training restarting on Monday.
In each case the back-up oxygen supply (a limited supply of compressed air in a bottle) kicked in, but nevertheless, hypoxia can be serious.
F-22s previously had a hypoxia issue, with one pilot being killed when he either blacked out or became disorientated and crashed in Alaska, but in the F-22's case at least one major cause of that was faulty hoses and valves in the cockpit / on the pilot's flight suit.
On the F-22, T-45, Hornet, Super Hornet, Growler, F-35 and some other jets, they use a system called the OBOGS (On Board Oxygen Generation System), which takes compressed air from the jet's engines (before fuel and fire is introduced), cools it, feeds it through a contaminants filter and then injects it into canisters where a fancy molecular membrane absorbs nitrogen, creating a ~95% oxygen mix that the pilot breathes.
Air forces moved to this system from liquid oxygen (LOX) bottles because jets could only carry a limited supply of LOX, because LOX is very dangerous to handle and because the pure oxygen could cause pilots to have their lungs collapse from absorption atelectasis.
On the T-45 and the Navy's Hornet family, they're having issues (combined, there were >200 incidents last year) with the OBOGS where it's believed that contaminants (fuel, lubricants, hydraulics, coolants, etc) are leaking out of the jet, getting sucked into the engine and then either getting through the contaminant filter, or perhaps partially clogging it.
With the hypoxia being either limited to, or concentrated at Luke AFB (there's no word if any F-35 hypoxia events have occurred elsewhere) and only spiking last month, it's possible that this may be a training issue or a bad batch of parts delivered by a sub-contractor. Either way, we'll find out more in the coming weeks or months.