Monarch - a rare sight in Calgary

This must be one of the fastest photos I've taken! Usually, we don't see Monarch butterflies this far north in Alberta. I've only ever seen one before - it was so far away that all I could tell was that it was a large, orange butterfly, but a naturalist who was also there, said it was a Monarch. Yesterday morning, a few of us saw this one on Nose Hill. Amazingly, it flew down and stayed there for two seconds - no time to get closer or compose the photo - just click! I'm happy to have the shot, though. Apparently, Monarchs this year have made it as far north as Edmonton (a 3-hour drive north of Calgary), Alberta, and beyond, a very rare event. I wonder why this is happening.