anaerobic bacteria

It depends, but in general, it means you have a pocket of substrate that doesn't have enough water movement to provide oxygen rich water. Some anaerobic bacteria are harmless, while other break nitrates down into potentially toxic products. If the pocket is left alone, eventually bacteria layers develop around it to process the by-products safely. If these layers are disturbed, it can result in toxins finding their way into the water column.

It's hard to determine how often this happens, but if a sudden catastrophic event follows right on a cleanign which involved moving decorations that cover substrate, it's a possibility.