It is a very old saying that anything is possible so it means that nothing is impossible so whatever security level you have it can be broken by hackers. The reason can be different in almost all cases like in some hack it happens due to insider cheats, some exchanges hack their own site as they are running in loss and to shut down the exchange they do this. Some hacks are really done by hackers who are trying for long time and in the end they get success and hack it.
So their is no security which can be breached, it is only the users who have to be safe and alert by using lot of security steps.