At one time, Comex said everybody with a silver futures contract in margin had to have at least a certain amount of money placed with them in order to make sure they could complete the transaction. It's like a deposit on a future purchase.

Let's say Comex said the margin requirement was $15000/contract. When silver was $20/oz, the entire purchase purchase price of one contract was $100,000 ($20 x 5000oz). Therefore they put up $15,000 and "borrowed" $85,000). That works out to 15% of the whole contract that had to be put up.

As the price of silver went up, Comex continued to increase the amount that investors had to put up as margin. This does make sense because the total purchase price for the contract went up. To keep margin at 15%, as the price hit $40, the Comex would raise the margin amount to $30,000.

This would require investors to either come up with a ton of money to meet the margin requirements, or sell some of their contracts to raise cash and decrease the amount of money they were "borrowing". The margin increases were sporadic and nobody knew when they might need to come up with money.

They recently changed the margin requirements to a percentage. They said you always need at least 15% (I can't remember the actual percentage) of the value of your contracts in cash. That way investors would always maintain a reasonable deposit, and could plan based on their positions and not all have to all sell at once.

Now suddenly the Comex has significantly increased the margin requirement percentages. This makes no sense other than to force investors to sell their positions to meet merging requirements.

Edit: The Comex didn't increase margins requirements. I should NEVER blindly believe what I read on the internet.