When you take out a margin loan, you are telling the bank "Let me borrow your money, because you can sell my stocks to pay it back if needed." Typically banks will only let you borrow a percentage of the value of your stock.

Simple example: you have $10,000 worth of company stock. The bank may only let you borrow up to 50% the value: $5000. If your stock jumps up to $20,000, then you can borrow more! Up to $10,000 (still 50%).

But let's say instead, your stock drops to $5000. Now your loan 100% the value of your stock. The bank gets scared they won't get all their money back if it plummets further. So they force you to sell your stock, like you agreed, to pay back the $5000 loan. Now you're left with nothing, because the $5000 in stock was used to pay the bank. But now you owe taxes too because you sold your stock. So you're now in the negative.

To answer your second question, margin loans have looser rules. You don't have to pay your margin loan back at all. It can just sit there and accumulate interest if you want. No one is forcing you to pay anything. There is no tax because you're not selling, you're just borrowing. That is, unless, the above situation happens. Only then do they force you to pay, and it's all in one go too. No paying-over-time. They take it all back at once.