Maybe the Google folks are more likely to apply somewhere with insane, multi-day interview processes that can stretch out over weeks, so the Tandem folks who wanted to leave didn't feel like they had to wait until they had a huge chunk of time off to look for a new job?

This is purely anecdotal, but I have a handful of friends who left google after their sabbaticals. They all explained it with some form of "I was surprised how much happier I was when I didn't have to go to work, and I didn't miss it at all." A couple of them actually chose to not work at all for a while, and one started his own company (a small lifestyle business, not a unicorn-bound startup). Using the time to interview and heading off for another full time job is not a pattern I've noticed at all...

I think Google was unique in that most of the people who were hitting their sabbatical had done very well with their stock options, and realized that they didn't need to work anymore and were quite happy not working, or working on their own stuff.