Ampersand at the beginning of a line in csh

It's backgrounding "nothing". The ampersand also functions as a command delimiter like ;.

You'll find that

; echo 'hi there'

does (in this case) essentially the same thing.

These, however, are different:

sleep 10 ; echo 'hi' & echo 'there'
sleep 10 ; echo 'hi' ; echo 'there'