When people buy at ATH, they usually hold it till next ATH. This way we get more holders and bitcoin price gets support level.
Also when price fall after ATH, people start buying in the hope of making some profit during next ATH.
This depends on that holder.
The price won't always be breaking or creating another ATH every month or every week. That means that when they will be investing during the ATH, that means there is a high chance that it will go down. Some people selling or maybe a correction happened where they could buy or reinvest. Saying that, it means that falling down is just good as going up.