What was a major event of overpopulation that was between the 20th century until now? (Ex: baby booms, immigration, etc.)
The Baby Boom is the largest cause of overpopulation in the world since WWII. Advancements in food and medical technology, as well as increased standards of living, have contributed to overpopulation in the Western world.
The population growth rate is double each 35 years. It has averaged double per 35 years, for as long as records have been kept. No single event has been a major contributor to that growth rate, on average. Each increase has been followed by a similar decrease.
There is, however, one way to reverse the rate. That would be mandatory sterilization after the second child. If you are father, or mother, to two children, take out the baby carriage, and, leave the playpen.
I realize, this threatens what most men perceive as their manhood. But, there is nothing manly- or womanly- about being a contributor to the suffering caused by overpopulation.