Bert Coules presented the following explanation :
And I shall do that. Thanks.

Better to make the negative connection last and make it first. It is
also better practice to make/break the negative connection away from
the battery, assuming the battery is connected to the car - connect the
negative of the charger, to the car chassis/engine block or etc.. It
minimises the chance of a spark igniting gas from a battery.

If you cannot connect away from the battery, always try to waft some
air across the battery, to dispel any gas.