There are a number of alternate explanations but we cannot rule out the relationship between the AL and Saudi government.
Regarding the substitution factor, even though Pakistani people might be sending more money per person, they are the closest to the Bangladeshi workers and can actually be a substitute. Pakistan is not exactly a very rich nation and they would be willing to do the same jobs that Bangladesh did. It is a large country with a large population, many of whom are extremely poor.
Another theory is related to the Saudi unemployment rate which is pretty high. Saudi government was really promoting and trying to push its people to work (they are generally considered lazy) and if necessary move into low earning jobs also. They raised salaries as well for Saudi citizens.
Asif Khan