Another reason behind the Friday fall may be that in the US, the last day of trading expiring stock options normally falls on the third Friday of the month. This is where stock options, index futures and index options all expire. Such deadlines can help create volatility, increasing the chances of large dips.
Share prices often rally ahead of long weekends and three-day holidays. They also tend to experience their biggest falls of the week on a Monday and their biggest rises on a Friday.