premise: Fan and Mortar Geysers are two geysers in the Upper Geyser Basin in Yellowstone National Park. For the past several decades, they have erupted in concert with one another and are generally talked about together. The records detailing these geysers' known eruptive history shows that they have been infrequent and irregular performers.
hypothesis: Fan and Mortar Geysers are unpredictable when they erupt. 
reason: "The records detailing these geysers' known eruptive history shows that they have been infrequent and irregular performers." They are unpredictable based on this sentence.