Warranty
You have two things going on there. The first is some serious galvanic corrosion, the second is a stress/fatigue crack. My bet? You had something made of mild steel that repeatedly sat in that position and came in contact with the rear crossbar in the track. Maybe someone used mild steel bolts/cotter pins or zinc coated bolts in place of stainless on the main sheet traveller, or you had something else... Nut/bolt/washer stuck in the track for a while, and collected salt water. Because aluminum is more reactive than steel, the aluminum corrodes twice as fast and keeps the steel from rusting. Ships place chunks of aluminum on their hull to keep the steel hulls relatively rust free underwater for this exact reason.
Once the aluminum in the track became thin enough, the crossbar developed a small stress crack originating at the thin section. Once started, it continued to propagate around the crossbar. Unfortunately, there's really no saving that one. You might try to weld it, but being that it's heat treated aluminum it will be significantly weakened. But the rear crossbar has significantly less stress than the forward one, so maybe it can handle it? That's for someone else to say, I'm sure someone's tried it.
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