Their net income swing is almost entirely attributable to a ~1 billion change in investment returns. Their operating income only fell 20%, because they had 8% increase in operating expenses without a commensurate increase in revenue.

It's actually all laid out in your link.

Curiously, the 1980's liability insurance crisis also arose out of dramatic swings in investment returns, not massive changes to the liability risk side (instances or awards). Which itself precipitated from the Volker Fed's exceedingly ill-considered moves to reign-in inflation (ironically because the financial / FIRE sector dislikes high inflation).

Insurers (and the Cleveland Clinic) achieve income through both premiums charged and investment return on those premiums. Because premiums are a competitive market, more subscribers can be acquired by lowering premiums, possible by pursuing higher investment income. Which works until it doesn't.

Premiums and payouts are locked-in contractually, whilst currency valuation and investments float on the market. Stay on the right side of that and you're Life's Golden Child. Get on the wrong side and nothing can go right.

Curiously, the insurance (that is: risk-management industry) seems blind to the investment-side risk and keeps getting bit by that bug every decade or two.

Wikipedia's (very brief, incomplete) treatment:

https://en.wikipedia.org/wiki/Liability_insurance_crisis

From these links -- revenues of $7.2b in 2015, $8.9b in 2018; money inflow sure isn't pinched.