The bonus exists to guarantee someone shows up. A liquidator must pay gas, take price risk while selling, and win a race against other bots, so the reward has to exceed those costs or the debt stays unpaid and becomes bad-debt.
Riskier collateral carries a larger bonus because the seizing and selling is harder. That also makes the cost of carelessness proportional to the risk of what you posted: being liquidated on a 15% bonus asset costs 15% of the seized amount on top of the loss you already had.
From the borrower's side this is simply a fee for ignoring a warning that was visible for as long as the drawdown lasted. From the protocol's side it is the price of solvency, and in extreme moves even a generous bonus may not be enough.
Related: on-chain-liquidation, health-factor, bad-debt, lending-protocol