VRLA batteries may not be expected to have a life of more than 5years.
The health of the batteries can be monitored by measuring the specific gravity of acid in case of vented type battery cells.
However, with VRLA batteries, the battery impedance monitoring (commercial devices are available)is the only available method apart from physical appearance like distortion, cell voltage on no-load / on load etc, which are not known to be all that reliable methods.
The manufacturing defect also can be an issue such as the cells gas more than they should while on charge thus causing excessive internal pressure build-up/explosion. The problem could also be with the battery charger settings.
Thus, the subject needs to be looked in to from different angles and I tried to present few of my thoughts.
Trust you find them useful.