Why, when crossing hybrids of the first generation with each other in the second generation, splitting of traits occurs?

First generation heterozygous hybrids produce two types of gametes. When they merge in pairs, four variants of descendants are formed. In the majority of offspring, 3/4 will retain the dominant trait of the parent, and 1/4 of the offspring will show the recessive trait of the original parent. Thus, in the second generation, a splitting of traits occurred. The splitting rule always holds.