Apart from the obvious, a shortage of food due to it being winter, therefore there could be a plentiful supply, magpies as I'm sure you're aware, are corvids.
I'm going to stick my neck out a bit here, so I could be very wrong, and hopefully someone will correct me, but I wonder if the magpies are juveniles learning to exchange ideas between each other.
A bit like PGL for magpies.....
I know crows do this, or something very similar, but I couldn't find anything to suggest magpies do as well, and like crows, magpies are social and like crows, magpies are very intelligent.
As I said earlier, I could be wrong, and hopefully someone will correct me.
Mike
Flickr Peak Rambler