The answer is One pea plant mutates to have a new allele.
Some of the conditions that must be met for a population or an allele to be in Hardy-Weinberg equilibrium are:
1. The population must be very large.
2. There must be no migration and population must be isolated.
3. There must be no mutations.
4. There must be random mating, which means there are no mating preferences.
So, if there are mutations in the population, the population will not be at Hardy-Weinberg equilibrium.