A freeze–thaw cycle is one round of freezing a solution and returning it to liquid. Each cycle exposes the solute to ice-crystal formation, to sharply increased local concentration at the advancing freezing front as water is removed into ice, and to time spent at intermediate temperatures on the way through.
For peptides the accumulated effect is degradation and aggregation, and it is progressive rather than sudden. This is a common explanation for a stock that behaves differently at week six than it did at week one, when nothing else about the experiment changed.
The standard mitigation is to divide a reconstituted vial into single-use aliquots at the moment of reconstitution, so each portion is thawed exactly once and the rest of the stock is never disturbed. Recording how many cycles a given tube has been through is cheaper than discovering the answer experimentally.