Regarding a 2nd height growth spurt. what are the causes of this? do they get a 2nd height growth spurt because their growth plates were still open even after 18 years of age?

Poor wording: I don't accept the 2nd "spurt" idea. There are some who naturally develop later & grow continuously until they stop even into the early 20's. The condition is called constitutional delay of growth & runs in families. Bone age (growth plates) remain open much longer than expected for age. A simple x-ray of wrist & hand & review of the growth pattern/chart establishes this diagnosis.