Hippos typically feed during the evening.
Often times during the day, they'll find a nice sandy place to lay around on if they are not in their pool.
Three hippos laying on a sandy bank above their pool (check out the snaggle tooth on the one to the right)
We occasionally will see one lonely hippo grazing during the day. It was quite unusual for us to see several hippos grazing directly next to their pool during the day.
Two hippos grazing during the day
A hippo pool with several hippos grazing on the bank
The migration is over and the wildebeest have successfully shortened all the grass. Perhaps the hippos aren't getting enough food during the evenings so they are having to supplement their intake?