Puffy SMC packs, what did I do wrong?

I bought a pair of SMC packs a couple years ago. Nice 2S 5 Ah hard packs, w/10 ga. wires.

Sometimes, I would leave them charged for a couple weeks, but not more than that. I ran them down, not overloading them. I would leave them discharged for a few days sometimes, but long-term, I always put them to storage charge. Now, they puffed up and I had to give 'em the ol' salt water bath and toss them.

Meanwhile, I had a pair of SPC packs that were subjected to EXACTLY the same uses in the same vehicles, are a few months older, and they're still doing fine. I looked to get some more and found SPC is out of the LiPo business now. :-(

Decided to try some Gens Ace and Tower Packs, but I would like to come back to higher end packs, as long as they will last properly.

Does SMCs use cheaper cells, or...?

I have a pair of Turnigy 2S 4200 40C packs (same dimensions as the old 6C NiCads, for use in vintage cars) that are starting to puff up, but it's a higher load on them, for the C rating, and they're cheaper Chinese packs, so I'm not surprised at that.