The buildup was that the challenger had bested the reigning champ in the first bout and now the champ had to make a comeback.
champ refers to a field, Bertin was likely the original owner of that field, and the name still holds today.
The reigning Emmy champ coming off her best season yet on The Good Wife, Margulies could very well set that record, too.
1868, American English abbreviation of champion (n.).
"to chew noisily," 1520s, probably echoic; OED suggests a connection with jam (v.). Earlier also cham, chamb, etc. To champ on (or at) the bit, as an eager horse will, is attested in figurative sense by 1640s. Related: Champed; champing. As a noun in this sense, attested from c.1600.