Cellular telephony does not use 2.4 GHz or 5.1 GHz frequency bands. My guess is either people's smartphones were getting on the courtroom's WiFi, as many smartphones do when they see WiFi available, or at least trying to connect, causing it to slow down from too much data load, or possibly people were setting up mobile hotspots as you suggest.
Wi-Fi uses open space as a medium and used 2.4GHz band with certain channels being used while communication is going on, if too many hotspots comes up in small space then it ultimately reduces the throughput in that space. May be this was the problem in this case.
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