A female Humpback mother and calf are often in the company of a third whale that is known as an escort. A Humpback escort may be a male or female whale.
If male the accompanying escort is usually just following the mother waiting for an opportunity to mate with her.
The escort is often seen defending the female Humpback against other male whales also wanting to mate with her.
The escort will actively try to discourage other whales from coming near her, this may include pushing and shoving or blowing bubble screens in an attempt to obscure the mother from the vision of a competing male.
Why a female Humpback would act as an escort is anyone's guess but there is safety in numbers and Humpbacks are sociable whales.