What Causes This Cloudy Glass in a Picture Frame?

Some friends of mine have a piece of framed artwork they’ve owned for ~20 years. At some point point (without apparent provocation) the glass became streaked / cloudy. The overall appearance is cloudy, a closer examination shows an almost spider thread intricacy. One would think the glass had been cleaned inappropriately and the cleaner had damaged the glass except the cloudiness appears to be on the inside of the glass, not the exterior. It does not appear to be liquid. Any ideas on what might cause this and the remedy thereof (besides getting a new frame/glass)?

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