Because my telescope's drive appears to think it's on the Earth's Southern Hemisphere, so the drive isn't tracking correctly. Ironically, even though this makes taking astrophotos a pain in the buttocks, the movement inspired me to make this into a single GIF. Feels like a flight over the Moon!
EDIT: Optics details: This image was taken at prime focus (no eyepiece) through an f/6.3 focal reducer, resulting in a focal length of 76.8" or 1951mm. Without the reducer, this telescope is an f/10, giving it a focal length of 120", which would increase magnification even more (at f/6.3, the Moon more than fills the field of view at prime focus).
Images are less than 1 second each; not sure exactly how long (I'll pay closer attention next time - was distracted by the combined image looking so blurry), nor of the interval between them. Say, 1/10 second each with an interval of a few seconds?
Next time I'll also use a polarizing filter to help reduce the over-bright areas, and adjusting the clock drive will allow me to combine images for greater sharpness. Here's a still from the animated GIF (under the cut):