interference, distance, more clients on AP, client chose to remain connected to weaker AP rather than roam to stronger signal (they do that and it's the cleints decision), connected initial data rate unsustainable so dropped to sustainable rate...probably other reasons but that'll do for a start.
Be pleased you are connecting at 140! A solid reliable connection is far more important than maximising throughput.
===2nd thoughts==== After a reread I might be missing your intention. Please provide more info about the speed drop, ie what device or widget is reporting it.
90% of the time, the answer is Client SNR is directly related to Distance from the AP. Higher RSSI, closer to the AP, translates to highest throughput. Try analyzing your client link thoughput at near and far distances, and you will likely see this.