One possibile cause of the split would be if it was damaged when pulling it through holes or over a sharp object during installation. A very small scratch can cause high stresses if the pipe hardens with age.
If you get any more splits I suggest taking out the whole length to the extent that it is accessible. If something is weak it fails at the weakest part. When you remove that weakest part you leave other parts that are nearly as weak. If you want to avoid more breaks you should do that before you get any more breaks, which are probably inevitable.
There is an interesting aspect of strength of certain types of materials. Someone determined that the average breaking strength of a large number of samples of long pieces of string is less than the average breaking strength of the same number of short pieces of string. The reason is that there are more weak spots in the long pieces.