Two main factors that triggered the entry of women in public life, and more specially in the labor market, were the two worldwide armed conflicts that took place in the early decades of the 20th century: WWI and WWII.
As most men were absent in the United States because they had to serve in the army and leave the country, women were encouraged to enter the workforce, mostly by accepting the factory jobs that "their husbands" used to perform before. An example, was the very famous poster that became very popular during WWII and which is attached in this answers.
The participation of women in solving the problems that the society was facing was crucial, and in fact, US women had in their hands the subsistence of the US economy.