The fertilizer might have done it. Are they nobile type dens or phal type dens? The treatment of each is poles apart. Nobile type dens bloom up and down the cane and phal type dens bloom from a spike at the top of the cane. In the winter nobile dens like it sunny, cool and dry with no fertilizer and phal type dens like it sunny, warm, evenly moist, and fed regularly. Dens lose their leaves at this time of year, so that's not a worry. In the summer both types of dens like it sunny, lots of water and fertilizer.