The cracks are caused just by the plastic itself being subjected to changes in temp. It's a lot of stress.
Skimmers can freeze solid and be fine because they're open. Virtually no one in the world believes this but it can be checked out by Googling info on why frozen pipes burst. The key is that ice itself doesn't expand, but it does take up more volume. To many people that sounds like the same thing, but it makes all the difference. Picture how the water in the skimmer freezes. It doesn't instantly freeze solid, right? It starts at the edges and moves in. As that ice forms it takes up more space.
In a closed system like a closed glass jar in a freezer or pipes in a house that extra volume causes pressure to increase in the system. More ice, more pressure. Eventually the glass, pipe or whatever can't take the pressure and it blows. In a house the blown pipe almost always happens far away from the ice, inside the house where the pipe/water is warm. If you look at pictures of blown copper pipe you'll see it looks like an explosion, which it is. A glass jar will explode, a can of soda will explode like a bomb. On the other hand an empty glass jar, no cap, placed in the freezer can crack. Many times it won't, but sometimes it will. The crack is caused of temp change, not ice.
In an open system like a skimmer the pressure can't build, so no problem.
The reason this is important is because I've seen many threads where people refuse to look up the info and choose to believe the myth. I have no idea why, but most of the time that's what they do. That leads them to do all kinds of strange things, mainly adding valves to seal off stuff. And there is an unlimited number of people ready to tell them their on the right track and make all kinds of suggestions to "try". Now they have a closed system and now they have real problems. Many people really, really hate learning stuff. It's boring, it isn't cool, whatever. But 30 minutes of reading can save you hours of work.
Then there's people doing stuff, the skimmer doesn't crack so therefore whatever they did saved the skimmer. It ignores the fact that skimmers don't always crack.
Stress crack in the bottom is common because picture the entire skimmer getting bigger when temp is higher, and shrinking when temps drop. Flexing back and forth. The bottom plastic is what is stretched and compressed the most. Just like bending steel back and forth will eventually crack so will plastic. Some plastics more than others.
The other way you can tell why freezing water didn't cause the damage is because of how much water expands when frozen. It's a lot. That would have cracked your skimmer in two.
So imo the problem is the plastic itself. Manufacturers have a choice in the type of plastic they choose to use. They have to sell products. If one product is a dollar less than another product most people will buy the cheaper. So if a manufacturer can save a little on plastic and the injection process/mold, they do. We force them to choose cheap because we do too. Plastic just doesn't hold up.
This cracking of skimmers also happens in warm climates. Out here in Phoenix people blame the sun or heat...but it isn't. It's kind of strange because I think most people consider plastic weak, cheap. But for some reason we also expect plastic things to last a long time. There's a reason manufacturers only guarantee these things for a year or less. They know how they're made.
I've seen people weld plastic skimmers back together, glue patches, goop it up. You can get a few more years out of it, maybe not. Replacing them is a more reliable, If you have to do that consider making your own skimmer out of EPDM. That flexes so can take temp changes no problem, doesn't become brittle at low temps. Save some cash and you can make the exact size skimmer you want which to me is gold.
Here's my DIY for building your own.
