I have a theory that a lot of 'new' riders who have never used toe-clips and straps don't realise how tight clipless shoes should be at times. In 'the old days', if you didn't do the straps up tight, your foot would've flopped around quite a bit and may have even lost contact with the pedal. With clipless, however, the shoe will still be clipped in firmly even with loose straps, so when people start hammering they're getting foot movement in the shoe.

but what the hell would I know?