There are three different recognized methods of producing PEX pipe and they are classified as -A, -B, and -C. -A is the oldest, most expensive, and produces the most complete cross-linking of the pipe and produces the most flexible and strongest version. The other types can be good, are usually less expensive, but if there is a little error in the process, much quicker to exhibit problems. -B and -C came about by trying to make it faster and cheaper, and sometimes, you pay for it in the end - stringent QA/QC is required. When PEX is not fully cross-linked, it can fail under stressful situations. This could be caused by a slight temperature variation or chemical makeup issue hiccup during manufacturing. If it is exposed to too much UV before installation or after, it will be degraded and could fail. The fittings are not freezeproof, and those can fail. -A PEX is the only one that can be safely 'restored' if it gets kinked without having to cut the kink out and replacing with a fitting as required on -B and -C formulated PEX tubing. So, take this into consideration when choosing PEX. If properly produced, it should survive the expansion caused by freezing if it wasn't already degraded during either manufacturing or exposure to UV or other handling issues (like pulling it through a tight hole and scoring it along the length). But, as noted, the fittings probably won't survive if they also get frozen.
Proving UV exposure didn't weaken the tubing is probably tough. But, randomly splitting for no obvious other reason may be a manufacturing defect. The tubing could be analyzed to determine how well cross-linked the molecules are, and that would tell, but the test is likely expensive.