maybe lack of water,sometimes too much water can do that to cacti, it all depends on the cacti and everything,if you have not watered them lately then water them well and see how they do..maybe add some potassium , i dunno really my cacti did that a year ago and i ended up losing half of the cacti i had,the biggest ones sadly, and they had to be tossed as the ended up rotting and now I am left with smaller but healthy cacti...sucks, 20+ years of growth at least on the big ones that rotted...but I will be patient and let the smaller ones i have left grow....