Those appear to be stress or fatigue cracks in the tubes. If so, get the butter because the fork is toast.
As to the cause, it could have started with corrosion, or the stress of the fork being ridden while spring apart a bit, or any number of hard to pin down subtleties.
But a crack where you have it, especially in both blades is the second problem ou could have with a fork. (the worst would be similar damage at the base of the steerer).
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