Happens quite often; can't speak from firsthand experience (in my races, never had the fortunate occurance of being in a breakaway with over a minute gap with under a few kilos to go), but some of it has to do with desparation...either continue to work together and watch the gap slowly trickle down until the peloton swallows you up, or take your fate into your own hands and try and break away for a solo.

It's worked in the past...but odds are certainly against you.