Well, we can never know what's in a critter's mind but it's usually hunger or curiosity, I would think. There must be some scent, or maybe something else growing in the soil where the spikes were. (I assume you didn't put more spikes out recently, since it's winter?) Are they eating the spikes or breaking them up?
You might want to try using a pelleted time-release fertilizer instead. The spikes sometimes don't distrubute fertilizer as well anyway.
