The final brown stage is the completion of the nutrients re-absorption process. Best is to wait until a frond is totally brown and shriveled up small. If new fronds soon turn yellow and head for brown, you have a nutrition problem. You are not feeding properly. Don't let that fool you.
Food is the answer.
Bugs may also be the problem, but not nearly as likely as improper feeding . Poor drainage may also be the problem, but not if your sago palm is well established in its location. Newly planted in your soil or a pot, poor drainage is a suspicion. A final suspicion for a newly planted sago is that you planted it too deep . There are serious problems with a newly introduced scale insect here in South Florida.