I regularly see a planting of what I am sure is ‘Pink Grootendorst’ (regularly reverting to ‘P. J. Grootendorst’) which does this all the time. The picture is from 2016, but I see it every year.

Some do it all the time. I raised a seedlng which has since been culled due to disease, that did it MOST of the time. https://www.helpmefind.com/gardening/l.php?l=2.63439 Tequilla Sunrise only required me to walk past it with any source of nitrogen and it threw globs of proliferation with one or two rows of petals beneath them like colored doilies. I didn't have to actually GIVE it any nitrogen, just walk past it carrying some.

I know nitrogen causes it, in some roses. But this particular plant hasn't been fed in -- oh -- a good year. So I'd tend to think it was related to our current shifting weather patterns.
Or something . . .

But I know some people may never have encountered it, so I thought I'd share.

And, it's historic...Rosa centifolia prolifera , Rosa centifolia prolifera foliacea, and others...

LOL, Kim! Candice does this to me on a regular basis but usually after I feed her. Gdinie, your pink G is beautiful! Jeri, your GM Hat is so pretty. She is on the top of my list as a replacement, if one of my other roses don't make it.

BTW, I think it was because of you and Reverence for Roses (they are so nice!) that I got Alice Hamilton. She is still just a baby band but is just blooming away! What a pretty rose! Seems to enjoy this cooler weather. When I looked up who Alice Hamilton was, I am very happy to have this rose. What a woman!

Yup! That was the movie. Wasn't it great?

It does seem to be weather, in some cases. "Arcadia Louisiana Tea" does this every November/December and at no other time of the year. She waits until all the petals in the original flower drop, then produces clusters of proliferation buds like little bouquets. So, the proliferation never disfigures the original flower, as it often does in other varieties -- it just sort of appears like a surprise at the end. I tend to be annoyed and intolerant of habitual proliferators, but ALT's proliferation is low-key and more like a charming quirk or oddity during the slow season. These were from November 15, 2018:

My picture was from September. There are quite a number of ‘Pink Grootendorst’ in the parking lot of a nursing home around here. They do the proliferation trick quite regularly, but I doubt if they get fertilized much. I can ask.

Btw, ‘Pink Grootendorst’ does very well with minimal care, but I am not a fan. The greatest disappointment is its total lack of fragrance, but it also holds on to its spent petals way too long, and the foliage is unremarkable. Still, it is a workhorse: hardy and never any disease.