You’ve got me beat on the cause of the clicks.
They can be repaired effectively with the “Repair” effect and / or “Spectral Delete”, though it would be very time consuming.
There’s also this “de-clicker” effect that may speed up the process considerably: https://forum.audacityteam.org/t/updated-de-clicker-and-new-de-esser-for-speech/34283/1
The only instruction for that effect are in that topic. It is a “Nyquist plug-in” and installation instructions are here: https://manual.audacityteam.org/man/installing_effect_generator_and_analyzer_plug_ins_on_linux.html#nyquist_install