Not investing such a large percentage of their capital in long-duration fixed income vehicles all at once.
There's nothing wrong with going long on duration, it's a hedge against decreasing rates. The problem is when you go all-in on long duration investments and rates suddenly shoot up like they did, you now can't sell those assets without eating a massive loss.
An appropriate hedge would have been doing what every retail bond trader does, build a ladder. If they had simply bought a wider variety of say 1/2/5/10 year securities then they could have let the longer-dated ones sit and sell the shorter duration ones (and they wouldn't have suffered such a huge loss of market value that spooked depositors and started the run in the first place).
Yields on short-term fixed income securities were absolute shit, barely above 0%. If you've got a bunch of cash and nowhere to put it to work then even a horribly yielding MBS seems like a good idea, and the inflation monster hadn't yet come to roost making the Fed start jacking rates up far earlier than anybody would have expected.
Even at the time it should have been seen as a short-sighted move, however. It was obvious ZIRP wouldn't go on forever and rate risk would bite you in the backside, so I can't call it anything but careless yield chasing without proper risk management.
There's nothing wrong with going long on duration, it's a hedge against decreasing rates. The problem is when you go all-in on long duration investments and rates suddenly shoot up like they did, you now can't sell those assets without eating a massive loss.
An appropriate hedge would have been doing what every retail bond trader does, build a ladder. If they had simply bought a wider variety of say 1/2/5/10 year securities then they could have let the longer-dated ones sit and sell the shorter duration ones (and they wouldn't have suffered such a huge loss of market value that spooked depositors and started the run in the first place).