External flash automatically synchs my GH3 to 1/60

I mounted an external flash on the hotshoe of my GH3 today and noticed that it synched the camera to 1/60th of a second (in aperture-preferred mode). I was a little surprised, since this was a Sunpak 433D strobe with a Nikon shoe on it.

I bounced the strobe, which had a white card attached to the back of it with a rubber band, off the ceiling and got well-exposed pictures.

I hope I get the same results with my Nikon SB800 strobe.

--StevenN