[Android Q&A, Help & Troubleshooting] BestSignal:
As mentioned by others, the most likely culprit is high memory utilization. However, there are a few other reasons that may contribute to the application closing down. Android "ranks" applications from 1 to 5 (where 5 means it is the first to get killed) based on certain criteria. Because xda won't let me link to it (I'm a new user), I have posted them at the bottom of my message.

Chances are, you are seeing the behavior in numbers 4 and 5. The fact that Android keeps applications in a least-recently-used list means that if you have applications which you just accessed but appear to have closed when you come back to them, then, once again, it is likely you are using a lot of memory that the phone is aggressively trying to keep cleaned up.

Although, it is possible that a small number of your problems are based on poorly implemented applications since the developer website states "If an activity implements its lifecycle methods correctly, and saves its current state, killing its process will not have a visible effect on the user experience..."

1. Foreground process

A process that is required for what the user is currently doing. A process is considered to be in the foreground if any of the following conditions are true:

It hosts an Activity that the user is interacting with (the Activity's onResume() method has been called).

It hosts a Service that's bound to the activity that the user is interacting with.

It hosts a Service that's running "in the foreground"—the service has called startForeground().

It hosts a Service that's executing one of its lifecycle callbacks (onCreate(), onStart(), or onDestroy()).

It hosts a BroadcastReceiver that's executing its onReceive() method.

Generally, only a few foreground processes exist at any given time. They are killed only as a last resort—if memory is so low that they cannot all continue to run. Generally, at that point, the device has reached a memory paging state, so killing some foreground processes is required to keep the user interface responsive.

2. Visible process

A process that doesn't have any foreground components, but still can affect what the user sees on screen. A process is considered to be visible if either of the following conditions are true:

It hosts an Activity that is not in the foreground, but is still visible to the user (its onPause() method has been called). This might occur, for example, if the foreground activity started a dialog, which allows the previous activity to be seen behind it.

It hosts a Service that's bound to a visible (or foreground) activity.

A visible process is considered extremely important and will not be killed unless doing so is required to keep all foreground processes running.

3. Service process

A process that is running a service that has been started with the startService() method and does not fall into either of the two higher categories. Although service processes are not directly tied to anything the user sees, they are generally doing things that the user cares about (such as playing music in the background or downloading data on the network), so the system keeps them running unless there's not enough memory to retain them along with all foreground and visible processes.

4. Background process

A process holding an activity that's not currently visible to the user (the activity's onStop() method has been called). These processes have no direct impact on the user experience, and the system can kill them at any time to reclaim memory for a foreground, visible, or service process. Usually there are many background processes running, so they are kept in an LRU (least recently used) list to ensure that the process with the activity that was most recently seen by the user is the last to be killed. If an activity implements its lifecycle methods correctly, and saves its current state, killing its process will not have a visible effect on the user experience, because when the user navigates back to the activity, the activity restores all of its visible state. See the Activities document for information about saving and restoring state.

5. Empty process

A process that doesn't hold any active application components. The only reason to keep this kind of process alive is for caching purposes, to improve startup time the next time a component needs to run in it. The system often kills these processes in order to balance overall system resources between process caches and the underlying kernel caches.