A few bubbles while charging a full or nearly full battery is normal.
Vigorous outgassing that looks like boiling is definitely abnormal.
When I fully charge a car battery I take the caps off.
One thing I watch for (at the end of the charge) is an equal amount of bubbling in each cell.
That means the cells are still well matched.
Outgassing increases rapidly when charge voltage rises above ~14.5V, as some fast, "boost" or non-automatic chargers will allow.
Automotive and deep-discharge "wet" batteries can benefit from an occasional "equalizing" charge at 16V (8V for a 6V battery).
This is done for an hour or so and after the battery has been fully charged at a lower voltage.