At the risk of being hopelessly pedantic, I think we need to pin down what is meant by “chain reaction.” Dictionary.com gives:
— n
1. a process in which a neutron colliding with an atomic nucleus causes fission and the ejection of one or more other neutrons, which induce other nuclei to split
2. a chemical reaction in which the product of one step is a reactant in the following step
3. a series of rapidly occurring events, each of which precipitates the next
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So I think sense #3 applies to rapid crystallization once nucleation occurs. I recently saw a tv documentary showing supercooled liquid water in a glass jar suddenly and rapidly freeze once seeded with ice. I think the “ever-increasing pace” described by @thorninmud qualifies as a chain reaction in the most general sense of self-reinforcing. Sorry I don’t know anything about crystallization of proteins.