Second postback being raised automatically when the first postback is taking long (more than 3 mins) to complete its processing

Hi Everyone,
I made a postback to server by clicking submit button, and the server is making set of webservice calls (around 10), due to issue with some services it took more than 3 mins (say 5 mins) to complete all the service calls, meanwhile at around 3 mins from first postback ,  a second post back is being raised automatically( not sure who fired it) causing duplicate processing and resulting in failure. Can anyone help me to find why this second postback is being raised and who fired it..?
I reproduced the same issue another pages which are not having service calls at all, by introducing a sleep(System.Threading.Thread.Sleep(200000);) of 3 mins..here also after 3 mins from the first post back a second post back is being fired, and i am able to see logs under post back scenario being written 2 times (each one for each post back)..
Can anyone help me to understand this behaviour..? i.e Second postback being raised automatically when the first p ...