Batteries can be damaged by excessive heat. If you have been driving long distances in desert areas where it it really hot, like the Mojave desert or something, that might explain it. In any event, batteries are usually warranted for much longer than 18 months, so I expect you should be able to get a replacement battery for free or nearly so.
It’s unusual still. It could be just a bad battery from the get go. If you’ve added any electrical components to the car – such as an alarm system or stereo system – that could be the culprit too. Suggest to ask your mechanic to do a current drain test when everything is off and the computers have had a chance to go to sleep. Anything more than 20-25 mA would suggest a possible problem.