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Why did the battery blow out?

I have a bed for which the headrest can be adjusted. There are 2 separate power inputs for each side like in the picture:

![Image_alt_text](https://electrical.codidact.com/uploads/qvgg37vqo8grtx439bv06ymb0avz)

In addition, there is a set of 9V batteries on each side, presumably for backup:

![Image_alt_text](https://electrical.codidact.com/uploads/ficxhkyabh34x8oest801sjyh9vh)

Yesterday I heard a loud bang and found one battery on the floor:

![Image_alt_text](https://electrical.codidact.com/uploads/0qjo4l1vpc8psjt04lrhc1fel0ax)

By looking at the battery, is it possible to determine what happened to it? There has been no water damage or anything else I can think of. The batteries were old, I don't even know how old. Is this a normal failure mode for old batteries?