estrechaban
-they were narrowing
The word estrechaban is the imperfect form of estrechar in the third person plural.
See the full estrechar conjugation

estrechar

Cada vez que miraba alrededor, las paredes se estrechaban un poco más.
Each time I looked around, the walls moved in a little tighter.
En los últimos años hemos visto cómo se estrechaban las categorías cada vez más.
Over past years, we have again and again seen the categories narrowed down.
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