It is because you re using too many applications at the same time.
it still happens even though you are not playing music nor sending a mail.
whenever you run an application (only apple's) and when you click the home button to return to the home screen. It doesnt really quit the application, but remembers where & what you have used for the last time. Thus when you click on the particular application again, you will realize that it still remains at where you have been using last time.
When this happens on many of your applications on your iPhone, it turns out to use up more of your memory, by not releasing it for you to use other applications.
This happens on mobile safari, and other applications as well, but safari uses up the most memory and definitely crashes when the phone doesnt release enough memory for it to run for a long period. Should you notice that iPhone's safari is a full featured html browser, where none of the other mobile device in the market does. It is actually normal when it crashes ones in a while, but it doesnt really happen that often.
Every iPhone user faces this issue, and it is not the user's problem nor apple's mistake. A person cannot expect too much from a phone which is already powerful enough for him or her to use.
If you tend to force quit every single application by clicking the home button for 6 secs, you might as well reboot your iPhone. And when you have been using many applications for a long period, you will notice that powering off your iPhone might take a little while longer than it does in usual. I hope this helps you =)
Thanks for ur help..
i try to restart my iphone like other forum mention and it does help,i on back the same website
that make me suddenly off..and it work..