When i go to the PageTwo.aspx page and then click the back browser button it goes to the PageOne .aspx.
I guess that "click the back browser button" does not request the PageOne.aspx but get the contents previously obtained from the cache of browser. It means that because no request was sent to the server by "click the back browser button" nothing (including the Page_Load) happened at the server side.
I suggest that you try to control cache by adding code such like:
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetExpires(DateTime.Now.ToUniversalTime());
Response.Cache.SetMaxAge(new TimeSpan(0, 0, 0, 0));
The above will generate the following response headers:
Cache-Control: no-cache
Pragma: no-cache
Expires: -1
Note that the above is only experimental try to confirm if the cache is issue.