the rotors are taper-fit. In other words, the bolts only secure the taper - they don't hold it on. Once the taper locks, it's a real bitch to get them off. Rotors tend, however, not to lock due to heat expansion, but the still do and can. I've changed about a dozen and only one has ever locked.
The professional way to break a taper is to use a puller. These are generally application-specific, however. Plenty of WD40, a rubber mallet and an hour or two banging should do the trick. Just remember to give the taper a wire-brush-up before refitting the new ones.