The BLS divides those who work part-time into two categories. Those who have to work part-time are counted here as doing so for economic reasons. Either their employer limits their hours to less than 35 per week, or the only work they could find was part-time. So, they are here because they have to be.
This chart remains for me the single most important chart of the past decade. The move you see in the middle has no precedent. The effects that it has had on work and pay are still being measured. The fact that we have now gotten back to the pre-recession level is very important.