95% of the time it is:
- Bot purge and along with that is removing any watch time generated by those bots
- reporting lag, and usually those resolve within 24-72 hours, but can take up to a week.
- YouTube suspects the channel losing watch time was trying to manipulate watch time and has adjusted it
This is almost always the answer. You will find this is a frequently-asked question with many responses. Search can be your friend in these forums.