This seems to happen with hormone therapy and with prostatectomy. (I don't know if radiotherapy alone can cause it). I don't know the cause, and neither does any urologist I've mentioned it to. Possibilities might be lack of frequent erections for a while, possibly urine leakage, or loss of Testosterone. If it's any of those, it pretty much must be caused by more than one of them (not in the same case), because just one doesn't explain all the cases. If the cause was known, there might be some preventative action possible.
There is some suggestion that forcibly stretching might make things worse, because it causes micro scarring which leaves the tissue even less elastic. Stretching with a steroid cream might reduce this, I don't know.
It does also happen with men who've never had prostate cancer treatments, but it does seem frequent after prostate cancer treatments, although I have no stats to know if it's really more frequent, or maybe just we're all so open we're more likely to talk about it.
It shouldn't be ignored because it increases the risk of acquiring Lichen Sclerosus which in turn is a risk for penile cancer (albeit rare).