Imagine that the peanut butter is not poured from the top, rather coming from holes at the bottom of the cup. Initially there's a radially growing patch of peanut butter around each hole. When the peanut butter patches grow large enough, they touch, and form an approximately straight boundary (if the flow rate is equal for all outlets) like the boundary between crystal domains.
Of course, peanut butter is a fluid, so its dynamics are ultimately different from crystal growth, but at this scale, it produces a very similar phenomenon thanks to its high viscosity.
Imagine that the peanut butter is not poured from the top, rather coming from holes at the bottom of the cup. Initially there's a radially growing patch of peanut butter around each hole. When the peanut butter patches grow large enough, they touch, and form an approximately straight boundary (if the flow rate is equal for all outlets) like the boundary between crystal domains.
Of course, peanut butter is a fluid, so its dynamics are ultimately different from crystal growth, but at this scale, it produces a very similar phenomenon thanks to its high viscosity.