Imagine Carman Senior leaves you his £20M international car recovery empire. It is made up of many fixed assets which cannot easily be liquidated, most of the value is the good will and strong brand and performance of the company derived from the balance sheet. Previously you may have qualified for some exemptions (e.g. BPR), but now we have new leaders with new ideas and they want their money. You're a high earner, so the government decide you should pay 45% IHT. You cannot find a buyer for the business and instead opt to defer the tax. Word gets out that Carman Junior is not the business mastermind that Carman senior was, costs mount, customers walk, suppliers take the pi** and slowly the value of the business falls. After a few years the business is worth £12M. The problem Carman Junior has is that he owes 45% of £20M + interest at 7.5%.
I think you can manage the sums to see that you are short a few mill.
Now imagine a software company or high tech start up, these firms can drop 50% in a few weeks.