Only way this could happen especially since you had it for a couple weeks extra in the secondary, would be too much priming sugar or any late additions of sugar after primary fermentation.
It is possible the yeast stalled and woke back up during bottling, but I highly doubt it.
I brewed a "cream O' 4 squash recipe" recently, with whole spaghetti, accorn, butternut, and pumpkin. I roasted half and took the other half and boiled it till tender before mashing. Got the idea from Hair of the dog Brewer when he does the Greg. He explains that way the grains help break down the starches to sugars the yeast will consume. Came out great with no bottle bombs.
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