When the loss of free childcare (on earnings over £100k) is added to the 62% marginal tax rate, and pension deductions, and maybe even some residual student loan deductions for younger ones, it's quite possible that they end up worse off by doing an extra day/shift, hence it's simply not worth it.
We also had the lifetime pension punitive tax cost, too, which meant that the likes of GPs were hit with a tax bill when their pension fund value breached the lifetime limits, which again, encouraged them to reduce working hours/days so as to reduce the amount by which their pension fund value increased.
Even if they're not "worse off" by working full time or extra days/shifts, there's a balance as to what the extra is worth compared with the opportunity of having an extra day off to allow them to do something else. When you're already earning well, you're not "desperate" to earn more as you can cope fine anyway, so if you only "earn" say 25% of the day rate for working an extra day, you may well think sod that, I'll have an extra day off - after all, it could mean they save on childcare, or dont' need a gardener, or don't need a cleaner, etc if they have a day off, meaning more savings!