Popcorn ceilings can be a grab-bag of fun and surprises.
Spraying- sometimes the weight of a healthy coat of primer/paint alone can cause a popcorn ceiling to sag in sheets.
Sometimes, although it may not be apparent by looking at it, some areas of a popcorn ceiling may not even be adhering to the sheet rock/plaster to begin with. Apply too much pressure with a roller and you break the brittle bond of the section that isn't adhering with the areas which are....it rolls off onto your roller in sheets.
etc etc etc
IMO, the best scenario is to spray a light coat of oil based primer...not too weighty. Then roll a coat of ceiling paint, while avoiding over-rolling...hit one section and move on. The oil primer offers a seal so the waterbased paint can't break the bond of the popcorn texture to the ceiling.