It can occur in a couple spots. One is transferring from carboy to bottling bucket. Couple things to look for -
1.) Make sure seal is good from your siphon to any tubing - sometimes airbubbles will get sucked into a line while transferring. Make sure tubing is tight.
2.) Make sure hose goes all the way to the bottom of the bucket - no "drop" of liquid from hose into bucket - no splashing.
3.) When you start filling bottling bucket - tip the bucket so that one corner fills up deeper faster, and make sure the hose is under the beer, as it fills, you can level it back out - but, basically, you want as little disturbance as possible.
4.) If you keg, and have CO2, purge the bucket first with CO2 so there is no O2 in the bucket.
5.) Same as above if transferring to a keg.
6.) If bottling, make sure tubing is tight and not letting bubbles in between spigot and tube, or tube and bottle filler. Also, tilt bottles to make sure one corner fills up first and disturbance of liquid/splashing is kept to a minimum.
Those would be some things to watch.
Congrats on the Competition. I had brewed for a long time and just started entering some this year - like you, I got some good feedback that has helped me fine tune things in my beers.