While trying to update a game I noticed that the download would start out estimating about 20 minutes until completion, but would gradually increase to about an hour and a half and then stay there. The only thing that changed was the "Disk Usage" shown on Steam's download page would drop from several Gbps to 100 mbps or less and sit there for most of the download. It would spike back up to 2-4 Gbps for a few second at a time then drop again. Is this a normal occurrence I just haven't noticed, or is there an issue with Steam or my computer?

In case it's relevant, I have a decent SSD with 200+ GB free and the file was only about 12 GB.