AGM's are subject to sulfate crystal growth internally, similar to flooded cell batteries...
This expansion over time, due to overcharging, or sitting idle in a discharged state ultimately results in the swelling of the battery case and then splitting on the corner ends - something's got to give...
The expanded state of these batteries can be a real pain when they're installed in a tight-fitting steel case, such as a 'UPS' (Uninterruptible Power Supply)system - to remove them can take considerable EFFORT!
Charging at a higher than recommended voltage will cause the one way vent valves to emit fumes - not a normal concern if the charging equipment is paired properly with the AGM battery bank...When this happens, it's usually the end of the battery's useful life...