The short answer? Since the credit crisis started. 
I'm sorry that you've become one of the many people who have seen adverse action due to the current credit climate. Lenders are tightening their belts, and borrowers (even the best ones with stellar records) are suffering the fallout. Believe me... it's not you. If you peruse the threads, you'll see that it's rampant right now. CCCs are chasing balances, ratejacking, reneging on 0% offers, CLDing (as in your case), and even closing accounts that have been inactive for too long. HSBC is known for quickly CLDing and closing inactive accounts.
Is this your oldest card? If so, stick it in a sockdrawer, and use it to buy a coffee or a pack of gum once a month or every other month, then pay it off immediately... to keep it active. If it's been inactive for more than 3 or 4 months, you might take a temporary FICO hit for using it again. But, you'll recover the month after.
The bottom line is that a lot of lenders are nervous, needing to reduce the liabilities on their books, and quick to take adverse action. Credit can be a 'use it or lose it' game.
One possible option is to try one of the backdoor numbers for HSBC in the "Backdoor" thread to try to get your credit line reinstated. Failing that, make them work to keep your business. That pack of gum, slurpee, or coffee (once every 1 or 2 months) is going to cost them a lot more than it will cost you.
