Need a little bit more information, like whether you had been drinking or taking any other contaminants into your body. These can influence recall. Generally though it has to do with neural pathways. Each brain cell has its own pathway or link to other brain cells. Sometimes when we are not using the information contained in one area very often its link will recede and the information will be cut off from us temporarily. It is believed that we remember everything that we do, have encounter and experience. However I think there may be a limit to how many pathways we can have active at any one time and so some memories can be forgotten for long periods of time. This also goes a long way to explaining when people get a blow to the head they develop amnesia as these pathways are most likely broken with the trauma. It also explains why people in comas for long periods can develop amnesia as they haven't been using there neural pathways. A pretty general explanation, and im sure someone could go into greater detail.

Since you use the ATM often, it means that the PIN is clearly in your Long Term Memory (Hippocampus). However, because of its massive capacity to store a vast amount of information, it isn’t always easy to remember things spontaneously unless you put them in categories.The Short Term Memory, on the other hand, serves as a temporary store for new information and also acts as a relay station for LTM; basically accommodating that bit of the LTM eg: your PIN, within its tiny capacity. Perhaps, you had a different thought while you were at the ATM; this thought was in your STM which basically shoved out the retrieved PIN code or the process of its retrieval. Usually, it works the other way…you tend to remember the retrieved bit and forget the new thought since it has never been anywhere near your LTM. But what’s significant is that your simple thought processes eat up the capacity of your STM and there is a limit to how many thoughts one can hold on to at a given instant[1]. I suspect that was the case for this reversed phenomena; you were definitely zoned out on something else…the fact that you remembered the PIN later is only because your STM cleared itself of enough thoughts to allow the PIN code to surface : )1. Miller, George A. (1956): The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information. In Psychological Review, 63 pp. 81-97Also referred to: Lahey, Benjamin B: Memory. In: Emily Barrosse. Psychology an introduction. 9th edition. McGraw-Hill press; 2007. P 238-9

In the global financial crisis, financial institutions are experimenting with a new security device which makes it difficult for anyone to actually get to their own money. The current versions uses a short range amnesic force field which causes those close to ATMs to temporarily lose key information such as PINs. One flaw in the current system is that it is somewhat age dependent. For reasons not yet clear the effect is more prononced on those over 50 than than younger people. Despite this drawback it is believed that the new device has saved banks billions in the short term.

I do like Kubra's explanation, bit like clearing out your cache.

There's more to what we do or do not remember. I have noticed several times when I have a dream, contradictory information that would make me realize that its a dream is not remembered. For example in one dream, I was living in a house (different from what I lived in in reality). Then I went to an office and filled out a form in the dream. Could not remember my address for the form.Similar stuff has happened several times in the past.