I believe it's a buffering effect. The speed goes fast until the transfer buffer gets filled, then has to wait until that data finishes getting written and it empties again. I get the same effect with large transfers to usb devices also.
I wouldn't worry about it. I don't think anything you can do to change it that would affect the total transfer time in any appreciable way. But if you want, you can try to mount the device with the "sync" option, which means that data will be written directly to the disk, instead of being buffered. You'll probably get a steadier, but slower, transfer that way.