What are "ha'Mabros"?
Rashi: They are places to cross the water. 1 [The chasers] thought that they returned to Arvos Mo'av, and the Yarden was in between.
Radak: There is a Shva under the Beis; it is from the root Me'aber with a Tzeirei. If not, there would be a Kamatz under the Beis, from the root Ma'avar (passage).
Why does it say "Al ha'Mabros"?
Who closed the gate?
Rashi: The gate-keepers.
Radak #1: The spies. 1
Radak #2: People of [Rachav's] house.
Malbim: The chasers, so if the spies did not yet leave the city, they will not be able to leave.
Why does it say "Acharei Ka'asher"?
Radak: Had it said Acharei Asher Yatz'u, it would imply after [a long] time. They did not delay closing it, due to their fear. Had it said Ka'asher Yatz'u, without Acharei, it would imply immediately after they left. Had they done so, the chasers would have sensed that the spies are hiding there! Rather, after the chasers distanced from the gate, to the point that they would not notice the closing of the gate, they closed it. This is why it says "Acharei Ka'asher."