The butterflies were fighting.
This came as a surprise to me. I had always thought of butterflies as being cooperative and peaceful creatures, but it seems that they can have conflicts.
This fight went on for ten to 15 minutes. I chose only three pictures to show the fight which seemed to be all accomplished with the forelegs.
There were two participants, a Canadian Swallowtail Butterfly (I think) and a Pale Swallowtail Butterfly. The Canadian Swallowtail was puddling, an activity where male butterflies sip nutrients from the soil which are then used in procreation. For the Pale Swallowtail to have attacked it, the source of nutrients must have been strongly confined. I suspect that it was urine deposited by a deer.
The Canadian is on the left with its proboscis busily extracting nutrients, when it was attacked by the Pale.
They have switched sides in this and a subsequent image. The Pale is attacking the Canadian, which continues to puddle.
The Pale tries to push the Canadian from its position, but it persists in puddling.