There are several potential causes for rotten bulbs. The most likely reason is that your soil doesn’t drain well. Most bulbs require well-drained soil; otherwise, they will suffocate and rot. Were the bulbs planted late in the fall? If planted too late in the season, bulbs may fail to develop roots before the soil freezes. Lack of snow cover may allow cold to penetrate deeper than usual and lead to frozen bulbs that rot. Basal rot or other bulb diseases could have caused the rot on your bulbs. If this is the case, avoid planting bulbs in the same location for several years. Another possibility is that you planted bulbs that aren’t fully winter-hardy in your area. If they can’t withstand the cold temperatures, they will die and turn to mush.