Why do Online Banking systems crash / collapse?

Online bank systems such as nwolb.com have come under scrutiny in the last few years due to the amount of maintenance that the system has had to undergo. The reason being why these types of systems have had to under such a large amount of re-structuring in the last few years and months is down to the fact that on average, each day, these types of systems see up to 1,000,000 potential entry break points of unknown hackers trying to break the system. The security team behind something like the natwest online banking system, which has been running for a number of years now, must be extremely experienced and knowledgeable in being able to ward off any potential hackers, otherwise, the back could find itself in a sticking situation, especially if there has been a considerable amount of money taken from a number of large scale investor accounts, which sometimes does happen.