Here are again pictures

Here is a link that might be useful: pictures

Now, are all of the leaves doing that, or just some of them?

While I'll agree that the ones with radiating spots definitely look like a disease of some kind (give it a hit with a broad spectrum copper based fungicide, see if it helps), its not unusual for older bottom leaves to start turning brown and dying off as the plant gets bigger, since they no longer get as much light and aren't needed anymore.