He borrowed $12.5B with Tesla shares as collateral when this condition is triggered he has to pay the loan back in advance. " Share Price Event: The VWAP of the Shares on the Exchange on any day is below 40% of its official closing price on the Funding Date" [0]

I think Tesla is creeping closer to this limit, but whether that is the actual reason... I don't know.

[0] https://www.sec.gov/Archives/edgar/data/1418091/000110465922...