began begining

[redgiant]

At six o'clock we were beginning to have supper.

Why does it use "were beginning" instead of "began" here?
I can't get the subtle difference between "beginning" and begin.
Is that the moment of "beginning" shorter than "begin"?

[Tdol]

If anything, it's longer; it's not just the first mouthful of food, but the wider context- putting things on the table, serving, talking, etc, and, presumably, something occurred at this stage of the evening, so the supper is the background to the narrative.