After mixed dentition is finished and the teeth become all permanent, The relation between molars on the end becomes class 1

After mixed dentition is finished and the teeth become all permanent, The relation between molars on the end becomes class 1 , why?

a-growth in the retromolar area ***

b-growth in the tuberosity area 

c-mandibular teeth drift more 

d-maxillary teeth drift more mesialy.