Tyre Clusters

I wonder why expired & discarded tyres seem to occur in clusters. Does someone carry their tyre along until they find a suitable congregation of others?

tyre_cluster_1_IMG9185-2013-05-02.JPEG

And who chooses the spot for the pioneering first tyre?

tyre_cluster_2_IMG9186-2013-05-02.JPEG

In the Donkey Field off the North Road, Ards.