There might be nothing unusual about this at all. It might be as simple as that Chubb have demanded some VERY extensive collateral.
DB clearly was hurt, because they missed out on profits.
However, you can also argue that it was DB's responsibility to do a sense check of Trump's numbers. Indeed they did do this and adjusted the terms of their loan accordingly, it's just even applying that correction they still overestimated his assets substantially. Nevertheless, it can be claimed that was their responsibility and fault.
DB is probably not going to enter a court and say it thought it was diddled, though. Firstly, it might genuinely have been okay with the loan it made. Secondly, it's just embarrassing for DB to admit it was hoodwinked or didn't get its sums right, and it's not like the bank is going to get anything unless it's getting the money. Thirdly, everyone can see the polls and that Trump has a very good chance of being the next president. What major bank is going to upset the next president of the USA - with risk of reprisals - when it doesn't have to?
Allegedly, DB decided to stop doing business with Trump long ago, but for some reason that remains very murky, one of their high-ups in the NY office decided to ignore the memo. Allegedly, there was a lot of unhappiness in DB HQ about it, but once they found out, they couldn't take it back.