[JshWright]

They need to launch when the orbital plane of the station is directly overhead. The earth rotates surprisingly quickly, so even a minute or two later the plane of Dragon's orbit would be far enough off from the plane of the station that a couple expensive inclination change maneuvers would be necessary to line them up again.

[rgbrenner]

The earth rotates surprisingly quickly

Not so much the earth as ISS.

The earth rotates at 24900mi/day or about 1040mph.

The ISS, to maintain its orbit, travels at 17150mph or about 5 miles per second.

So in two minutes, it travels about 600 miles.

[ohazi]

Yeah, but the window is short because you have to catch the orbital plane, not the ISS itself. Earth's rotation speed is the relevant number here for figuring out how fast that plane sweeps across the sky.