That the occupiers in a particular development have sold off their interest in the property/??? or moves out enmasse to deliver vacant possession to the developer.
Eg: "The earthquake forced the people to move out of their village en bloc."

ist la lingua francaise, in other words french, for altogether

In some places, the land might have been under developed, and the units are old. Individually, the units can only be sold at low prices due to bad conditions and age.
The house owners can group together and sell all the units including the land to a developer.

The developer can then rebuild the property on the land.

Of course, the developer should pay the owners a higher price than what can be sold individually.
If not, then no point for en bloc sales.