Multitasking is truly supported by devices with higher end chip configuration( i.e.good clocking speed 1Ghz and or sufficient RAM>=500Mb ).
In simple terms is that insufficient ram will not be able to hold more then one application at the same time and less processor speed will take some time to load in app data for both the applications(although i think RAM is the main area to be concentrated for your multitasking purpose)
Due to limited resource the kernel has to decide which task to be given more priority either task 1 or task2.Based on it's decisions only the browser is temporarily shut and the message is loaded.Later when you load the browser back the kernel gives this as higher priority and it loads the browser back.(If you have multiple number of tabs with heavy activity going simultaneously then you have to decide which is of least importance and close the tab.)
Ex-:Say you have opened up to 4 tabs named Tab1-Tab4.
As you can see all are processor demanding tabs and will not work all at the same time.
So if you opened Tab2(browse video) and later switched to Tab4(Social networking). The browser will reload Tab4 due to higher inactivity on this tab and not close it.
Same goes with the switching between two different applications.The browser will load the page but not close the tab.
What i can suggest you is that either you use less no of tabs in the browser and then shift to any other application.(Clearing the cache memory may also resolve the issue)
Note:-If at all you still feel that there are only 2 tabs and still the browser has crashed/closed the tabs while in-app/switching through 2 different apps then you need to contact the support team of the respective browser.