This occurs when mobile Safari either encounters bad javascript, or code, as mentioned above, but mostly this behavior happens because you are surfing sites which depend too much on RAM. Remember, this iPod now runs a full operating system architecture under it's hood. The memory simple runs out, and the iPod OS is told to terminate the process in order to restore that lost RAM.
You may see this behavior, for example, when listening to music while trying to surf the internet within mobile Safari. The memory is used up, and the OS has to terminate one process to keep enough memory to run the other.
This issue isn't really present anymore (at 1.1.4) for me.
As posted above, lets hope for the fee Apple are thinking of attaching to firmware 2.0, that it fully addresses this issue and many others!
Hope this explains.
R-Fly