Why past progressive was used in this sentence?

2018-04-08 04:24:26

It's from Charlotte's Web by E.B. White.

The grass was wet and the earth smelled of springtime.

Fern's sneakers were sopping by the time she caught

up with her father.

Why author used 'were sopping'? Is it past progressive clause or 'sopping' is an adjective in the sentence? And is it correct to say:

Fern's sneakers had sopped by the time she caught

up with her father.