It's fairly common, in a strong series C round and beyond for founders to take money off the table, especially when the VC appetite demands it. Typically, founders can sell as much as 10-20% of their vested shares, which can be worth 10s of millions of dollars or more.
I can confirm this goes beyond the founding team, I've sold shares as a part of raising capital at the last two places I've been employed. I was an early hire at both and held the CTO title. Series C in 2014 and most recently series B at the start of 2018. I also seek out opportunities to unload my equity in the secondary market, but I'm usually taking a haircut there vs the premium investors that are looking for a bigger share will pay during a capital event.
I'm a bird in hand guy when it comes to equity at the fast-growing private companies I tend to be attracted to. I'm almost certain I'd feel differently if I had a larger stake or founder-level attachment to what was being built.
