Earlier tonight this flight left KSFO for CYYC, but was diverted back to KSFO. I live about 25 NE of KSFO and have enjoyed watching aircraft for years, since I was a kid, but this flight caught my eye as I was surprised at the low elevation downrange from the airport. Then about 5 minutes later I saw large aircraft flying in a very unusual pattern. After checking the tracker, i saw that it was indeed diverted back to the airport. Is there any way to see or a site that can explain why this occured?