The most likely cause of higher airtime utilization on the first 20 MHz of the 80 MHz channel is non- Wi-Fi interference. Non-Wi-Fi interference can prevent an AP from using its full channel width, as it will degrade the signal quality and increase the noise floor on some parts of the channel. This will force the AP to fall back to a narrower channel width, such as 20 MHz or 40 MHz, to maintain communication with its clients. The waterfall plot can help identify non-Wi-Fi interference by showing spikes or bursts of RF energy on specific frequencies or sub-channels.
The other options are not correct, as they do not explain why only the first 20 MHz of the channel has higher airtime utilization.