system
A chat on subreddit /r/wallstreetbets
human
The willingness to lend does not directly depend on T-Bill rates tho. 
 Banks generally fund themselves from deposits (pay no interest on some of them as well). On top of that, their NIM will depend on the credit spread that compensates them for default risk. 
 So while yield curve shape matters somewhat, it is a rather weak predictor of willingness to lend.

gpt
I would say that both willingness to lend and yield inversion/low long-term rates are byproducts of a larger event. And that is overall risk in the economic landscape. More risk leads to more conservative lending practices and more hoarding of treasuries.