If they're "expected to quit soon" because they seem bored with the place and they've been planning to move on to advance their career, they'll probably go without a lot of wailing and gnashing of teeth. Of course, if they're "expected to quit soon" because they're seething with rage and angry about everything, the reaction might be quite the opposite!
As to why lay off instead of waiting for them to quit, probably because you've been told to reduce headcount now, not at some indeterminate future date.
As to why lay off instead of waiting for them to quit, probably because you've been told to reduce headcount now, not at some indeterminate future date.