Why is the progressive form used here?

In February 1825 he married Julianne Thiemer, daughter of a property owner and well-established glove-maker in Seesen. Heinrich, a cabinet maker without property, a Beiwohner(boarder), was marrying up. Together, he and his wife raised ten children in Seesen, a small city of about three thousand people six miles down the mountain from Wolfshagen. (Steinway & Sons by Richard K. Lieberman)

How does marrying up work in this paragraph? Is there any difference if you use past tense here?

Solution 1:

The Past Progressive is needed here because the writer is detailing the sort of marriage and the consequences of such a marriage.

Compare:

He married her. He married up. They raised 10 kids.

-- and --

He married her. He was marrying up. They raised 10 kids.

You will notice that the first case (with the Past Simple, your alternative) seems to talk about a series of different actions.

On the other hand, when we use the Past Progressive, it is clear that we're talking about the same action in the first and second sentences.

Solution 2:

The Cambridge Grammar Of The English Language (p165) designates this use of the progressive as interpretative/explanatory, and exemplifies it as follows:

• When I said 'the boss', I was referring to you.

In the OP's extract, the author uses the past progressive 'was marrying up' to explain or interpret the significance of Heinrich Steinweg's marriage.