The BLS breaks out part-time employment into two categories. This one counts the people who have to work part-time. They do so because they can only find part-time work, their employer limits their hours to less than 35 per week either because of slack work or because the job is structured to be less than 35 hours per week.
Note what happened during the recession, and what has happened since.
The orange line on the chart marks the level the series had reached in December 2007, which is when the last recession started. We just pierced that level, almost 11 years after we went through it from below.
It remains to be seen whether this will have any effect on the current compensation dynamic.