First time I'd heard it also which is why I askeed the question. For the record, I'm not complaining, nor did I dispute it with the purser at all. Just curious.

Welcome back to the UK (I assume) pax agent and thanks for the reply. You're not blunt.

Your response kind of makes sense even if I find it hard to imagine a situation where a bag would slide backwards. Normally a bag would slide forward if the plane came to a sudden stop and hence (I thought) the metal guards to stop them from doing that. Also, in my case I was in the aisle with the bag under the middle seat and noone on the window. There could have been no obstruction even if the bag were to slide backwards.

At any rate, many of the rules nowadays I take as given and would like to think they're designed with our best interest in mind even if they don't always make complete sense.

Cheers.