1. A system for detecting a boiling event at a cooktop, the system comprising: a vision sensor positioned so as to collect imagery depicting the cooktop; one or more processors; and one or more non-transitory computer-readable media storing instructions that, when executed by the one or more processors, cause the one or more processors to perform operations, the operations comprising: obtaining a first frame of imagery and a second frame of imagery from the vision sensor, wherein the first frame of imagery and the second frame of imagery respectively depict the cooktop; identifying a first plurality of motion vectors based on the first and second frames of imagery, wherein each of the first plurality of motion vectors describes a change in position of one or more pixels from the first frame of imagery to the second frame of imagery; generating a motion vector orientation histogram that describes a first distribution of orientations associated with the first plurality of motion vectors; calculating a first dissimilarity score for the motion vector orientation histogram, wherein the first dissimilarity score describes a first difference between the motion vector orientation histogram and a first pre-learned histogram, wherein the first pre-learned histogram describes a second distribution of orientations associated with a second plurality of motion vectors identified from imagery depicting boiling liquid, and wherein the second distribution of orientations indicates that the second plurality of motion vectors identified from the imagery depicting the boiling liquid have orientations distributed in a relatively uniform manner; generating a motion vector magnitude histogram that describes a first distribution of magnitudes associated with the first plurality of motion vectors; calculating a second dissimilarity score for the motion vector magnitude histogram, wherein the second dissimilarity score describes a second difference between the motion vector magnitude histogram and a second pre-learned histogram, wherein the second pre-learned histogram describes a second distribution of magnitudes associated with the second plurality of motion vectors identified from the imagery depicting the boiling liquid, and wherein the second distribution of magnitudes indicates that a relatively large number of the second plurality of motion vectors identified from the imagery depicting the boiling liquid are clustered about a particular magnitude having a relatively small value; and determining whether a boiling event is occurring at the cooktop based at least in part on the first dissimilarity score and based at least in part on the second dissimilarity score.