The message "This app will now restart to free up more memory" is a trivialized, totally BS description of the raw truth: "This app/process has crashed... and needs to be restarted".
So for whatever reason your app or process has crashed ( you don´t tell us which app you used thwn it crashed), you´ll never know the real reason since you don´t get any detailed error descriptions. It COULD have something to do with low memory, but it´s not sure. I also get app crashes from time to time with the preinstalled apps, mainly SmartShare. I NEVER use the Browser, it´s slow, painfully lacking in featues and everything... also crashes a lot, specially when using Video... stay away.