Android forces each app to be with in 512MB memory, with some exceptions for 1GB ones. The browser will take a lot. Indie non-native apps usually take a ton of memory. Apps that play video or deal with large images also take a good amount just because of the content. So, it's not that difficult to reach maximum. 

+ Amethyst has a growing cache issue. We clean a lot of things your are not seeing anymore, but many events still stay and it grows until it explodes. For regular users this doesn't happen because Android usually kills Amethyst before the cache even passes 50MB or so. But for "pro" users, the cache might grow so much that it triggers its own destruction.

This is specially true if you see a lot of GIF profiles. Many animated profile pictures use 20-50MB. If there are too many on the screen, like on the Notification's little profile parties, it will easily pass over 512MB and then it's up to Android to decide to kill or not. 

And what to kill. Sometimes Android just kills threads and leaves things in an inconsistent state. But those are rare.