Stalking horse

I see these horses often, since they live in a field I walk through on my usual riverside stroll/blip hunt. Normally though they show no obvious reaction or interest in passers by but today I was surprised and slightly unnerved to turn round and find they were all following me at close range and closing in. I held my ground and took a few photos and then walked on, slightly more briskly than usual, to the gate, getting another Polo out of the packet as I wondered what could be behind their unusual behaviour.