The Berkshire Cow model is a pictorial representation of the Economic-Value Creation (EVC) metrics of a business over its last 10-years.
A negative BV would mean that Assets < Liabilities. When this happens, the Equity Owners owe to the business instead of the business owing them! You then have Equity-‘owers’ as opposed to Equity Owners. Who then pays for the assets? Only the creditors.