A news release came out from Bungie today revealing a brand new system of earning credits or cR. They are allowing a credit jackpot to be released to lucky players who have a good game completion rating. This is a new push from Bungie to it's players to encourage staying in the entire game and not leaving your teammates alone to lose.
Early reports are of the jackpots being anywhere from 4,000cR to 9,000cR. Here is a full report.
Halo Reach Credit Jackpots- What is a Halo Reach Credit Jackpot