Well firstly they were offering perps[1] which are a margined product. If you want to trade with margin on a regular exchange (like say you want to trade commod futures on CBOE or ICE or whatever) you contact a broker/dealer and put up collateral[2] to get margin. FTX was seemingly both acting as an exchange and as a broker/dealer so they were providing the margin and standing risk on the other side of certain trades either as FTX themselves or as Alameda (I'm not sure about this).

Margin has the effect of magnifying the effect of volatility in the value of a product on profit and loss. And of course crypto is extremely volatile to begin with (by comparison to other assets).

Since they were acting as a dealer they could lose a lot if their book of business was imbalanced (ie on net a lot of their customers have the same position) and the market moved against them.

Secondly they seem to have had very little equity as a reserve against losses and what equity they had seems to have been in this FTT token which was only backed by the exchange itself. So if the FTT token lost value they would lose all their equity and have no reserves to protect against losses. This happened when Binance tweeted that they were selling their (very substantial) FTT holdings. In normal finance your equity reserves are there for if you have a bad day and lose some money - you can dust yourself off and try again the next day. If you have no equity then your business is insolvent. Trading when insolvent is no bueno and if there are rumours that you are insolvent of course people pull funds, won't trade with you so you have effectively a "run on the bank" and all the problems you had before get worse because now you can't find anyone to trade with as you try to wind down positions to refund customers etc.

Thirdly there seems to have been shenanigans afoot regarding Alameda (the affiliated hedge fund) which seems also to have primarily been long FTT and a bunch of other similar tokens backed by not very much. It may be exchange funds were being used by Alameda for trading - certainly they don't seem to have been kept at arm's length as they should have.

Fourthly there have been allegations that customer funds from the exchange were somehow inappropriately used to cover losses either at alameda or at FTX itself. If this is not true, it's hard to understand why refunds are taking any time at all so make of that what you will.

[1] Essentially a leveraged swap on the price of a particular crypto. So one party will pay say 10x the difference between some reference price and the current price of the crypto and the other side will receive it.

[2] Assets with a certain value as security against losses. Generally you will be required to have collateral which is liquid (ie can be easily sold if need be without affecting the value of the collateral) and relatively risk-free (like treasury bonds). For collateral that is risky, the dealer will take a "haircut" off the face value of the collateral to account for the fact that they may not be able to sell the things for the market value. The market itself (and Alameda) had collateral (FTT and similar) that was on the whole neither liquid nor risk-free.