In mendel's experiment, why did wrinkled seeds show up in the f2 generation even though it they were not present in the f1 generation

This was due to the parent plants of the F1 generation being true breeding dominant and recessive respectively.

Wrinkled seeds are products of the recessive allele, and round seeds are products of the dominant allele.

This caused all of their offspring in the F1 generation to be heterozygous, and therefore still possessing the dominant allele. This caused the seeds to not be wrinkled, but round.

Then when Mendel crossed two of the heterozygous F1 generation plants, of the F2 generation plants were homozygous recessive, giving them wrinkled seeds.