The plates can overheat, and if its not vented properly it will explode
You will wreck thi s type of battery if you use a car charger, they are completely differant types of battery compared to a leisure battery.
A leisure battery is basically a glorified car battery with better plates in it to prevent the plates oxidising from sustained, long periods of discharge, whereas a car battery is designed just to provide a large, short term discharge when you start the car. The car battery and the leisure battery are vented to allow any gasses caused from the high current diischarge and also from the higher charge currents to disperse safely. These vents are the reason that with both a car battery and a leisure battery needs to remain upright to stop the acid leaking out.
The sealed lead acid battery is a differant kettle of fish altogether. It by it's name is sealed, although if there is a buildup of pressure inside the battery there is a safety vent to allow that to be released. Because it is a sealed battery with no way to top up the acid like you can with a car or leisure battery, that's why you must use a charger that has been designed for sealed lead acid batteries. In an extreme case, if you used a normal car battery charger the gasses could build up too fast for the safety vent to work and it could, in theory, explode. What would happen is that the acid would vapourise and escape through the safety vent and then your battery would have less acid and thus less capacity for holding a charge.
The differance between the two types of chargers basically is that the car/leisure type charger will use quite a high charge current initially and will use a constant voltage, ie 13.8V or for a quicker charge some may use up to 14.4V.
The sealed lead acid charger is a far more intelligent being, it uses a differant type of regulation, it uses a Constant Current charge technology. The charger senses what state of charge the battery is in and uses a constant current to charge the battery safely, using a lower amperage so as to not cause a build up of gasses that deplete the acid.
The best sealed lead acid chargers are "Intelligent Chargers", they have a 3 stage charging method to maximise the life of the battery while still having a short a charging period as possible.
Hope this brief ( yea right !! ) reply helps
Tim