'Leverage' in a financial context is debt. If you borrow money to buy an investment, that is called a 'leveraged' investment.
When you invest with borrowed money, your personal risk increases. This is because if your investment is wiped out, you can't walk away from the debt, you must continue to make payments - so poor financial performance can lead you to possibly need to sell other assets to avoid bankruptcy etc.
'Deleveraging' is the act of reducing your debt tied to the investment. It could mean a few things - if you had $100k in debt and $300k in investments, 'deleveraging' might mean that you sell $100k of your investments in order to pay off your debt, leaving your investment amount lower. It might also mean that you use your other cash/assets to pay down the debt, keeping your original investment intact.
If a particular market is crashing, individuals who previously invested with borrowed funds must decide when to cut their losses to pay off their debts. If fewer people have decided to invest with borrowed funds, then that means there is less 'available money' in the market to purchase those investments [also called 'liquidity']. So in a fair auction where individuals are deleveraging [either by choice or because their brokers have 'margin called' them, meaning forced the sale of assets to cover debts owed to the broker], you would expect that there will be less bidding pressure to keep the price of that asset high.
Because crypto valuation is not based on fundementals, high bidding pressure is critical to keeping sustained pricing - lack of liquidity quickly leads to crashes in the crypto market. For assets which have concrete methods of valuation, if you believe the market is under-pricing an asset, you can wait until that valuation is reached before selling. Crypto is not valued based on anything concrete, and therefore market sentiment becomes itself the only thing that matters [more than for any real asset class].
In conclusion - as fewer people make the insanely risky choice of investing in crypto with debt, less money is bidding up the price, and as liquidity dries up, the price will fall.