How do credit default swaps cause instability in Greece? Take a horse race analogy for example:
When you go to a horse track, you research the horses, you've seen how it has performed in the past, but more importantly, you compare relative performance. This horse may be good, but another horse may be better. However, when you place your bet on the third lane, your bet does not alter the performance of the horse in that lane! This is the same concept of credit default swaps.
From the book, Getting Started in Hedge Funds:
If these men and women were so powerful that they could control currencies and markets, wouldn’t they do so all the time so that they could always make money? No matter how much or how little money one has, no one likes to lose it. If the funds could truly control currencies or manipulate the markets, these massive losses would not have occurred.... The Economist magazine believes the reason hedge funds [and Wall Street Banks] catch flak for all of the world’s financial crises is ignorance.
Still not convinced? Read Swaps Not Responsible for Crisis, Greece Is and CDS not cause of Greek debt woe. In this second article there is mention of AIG causing a problem with CDS contracts, but the difference is that they took the other side of the trade with other firms, as insurance policies (first person). CDS contracts on Greece debt are third-party deals. CDS spreads on Greece debt are just contract prices that firms must pay to enter into default protection and simply reflect how risky Greece bonds have become.