How were Power Bank limits decided by airlines?
The FAA did tests after a recommendation of the NTSB issued after a fire incident.
Presentation of the results of the test.

Basically they measured the gas emission from thermal runaway in the cells. Which is the dangerous part, as it is the gasses could cause the explosions.
I cannot find the motivation for the 100Ah limit in this research.   

If you know how much gas is emitted from cells you can estimate how much cells you need for an explosive or dangerous mixture, in say, the overhead storage.