Sunspots, debris and/or a distant cloud are not completely ruled out: https://arxiv.org/abs/1608.01316
The leading hypothesis, based on a lack of observed infrared light, is that of a swarm of cold, dusty comet fragments in a highly eccentric orbit. Many small masses in "tight formation" orbiting the star have also been proposed. The changes in brightness could be signs of activity associated with intelligent extraterrestrial life building a Dyson swarm. The SETI Institute's initial radio reconnaissance of KIC 8462852, however, found no evidence of technology-related radio signals from the star.