"The reservists were preparing explosives to demolish two buildings in central Gaza on Monday when a militant fired a rocket-propelled grenade at a tank nearby, Rear Adm. Daniel Hagari, the chief military spokesperson, said. The blast triggered the explosives, causing both two-story buildings to collapse on the soldiers inside."
Best guess... trying to make building unsafe for habitation instead of completely flattened.
It'd be easier for civilians to know where to hide if the choices were rubble or standing structures. Weaken the roof supports and load bearing walls but leave the building standing and anybody who runs to a standing building to hide during a firefight or bombing still might die due to the blasts nearby collapsing the building.
(And in the future, a pile of rubble is easier to clean up than an unstable building still standing. So just in case the Occupation forces do leave, it will be a more costly project for the Palestinians to rebuild)