mongoexport on 2.0.4
When you are exporting to .csv file you should use --field/-f/fieldFile  option.

http://docs.mongodb.org/manual/reference/program/mongoexport/

Try this

mongoexport -d central -c alerts -q "{updated_at:{\$gte:new Date(1420936780121)}}" -f  --csv --out alerts_11_12_Jan_20151.csv