Effects associated with the dates of moving holidays like Easter are not seasonal in this sense, because they occur in different calendar months and different quarters, depending on the date of the holiday. The extent and nature of this seasonality can vary markedly between series. For example, it is especially prominent in the case of electricity generation, or agricultural production, but relatively insignificant for a series such as total New Zealand population. Its values are unpredictable as regards timing, impact, and duration. It can arise from a combination of sampling error, non-sampling error, unseasonable weather, natural disasters, strikes, etc.
Random fluctuations are the main cause.
While every member of the population is affected by general economic or social conditions, each member is affected slightly differently. So there will always be some random variation in the series. This is not a problem, as long as it is small. If it is generally large, then it can become difficult to quantify the other components. This is because the irregular component is measured as a residual in the process of seasonal adjustment.