What do we learn from "va'Yiz'aku"?
Radak: They gathered to chase after them. Because this is done via screaming, this expression is used.
Malbim: [They alerted everyone, and] also the people who remained in the city to guard it chased after them, and also people of Beis Kel, and they abandoned the city open.
Why is it written ba'Ir, and we pronounce it ba'Ai?
Radak: The writing shows that they aroused everyone left in the city to chase after them. The pronunciation shows that this was in Ai and its boundary (neighbors), like it says "v'Lo Nish'ar Ish ba'Ai u'Veis Kel" (verse 17).