i think we should restrict discussion in answering these questions only .... not transforming the nature of questions into some religious discussion....
1) When did it start ?
in Pak cricket .... i think this trend started in early eighties or late seventies although it was present in bits and pieces before that ...
but the real trend setting of doing sajda for celebration came from our beloved sport .... Squash ... it was the beautiful scenes of Khan's legacy which glorified the whole world.... so every sportsman in Pakistan wished to repeat those glorified moments .... and the trend started .... its just like the glory of our fast bowling set trend of fast and furious bowling in Pakistan cricket..... n than this sajda from squash courts went to olympics ..... when our once powerful hockey team doing sajda in front of whole world .... n now in almost every sport in Pakistan you will see this trend .... from Squash to hockey , from cricket to football (remember final of SAF games in Islamabad) , aisam ul haq's in tennis, volleyball (when whole team perfom sajda after defeating india in wah cantt) , kabadi team performing sajda after defeating india , our base ball team performed sajda after winning the asian title (if i m not wrong ) and many more .... i think Jhara or Nasir bholo also performed sajda in wrestling .... Hussain shah performed sajda after his famous olympic bout ....
in World of sports .... no one knows .... many sportsman and sportswomen perform sajda or kiss the ground (non muslims) to celebrate their success .....
2) Are Pakistani players the only ones doing it ? After all Amla is probably more religious than the Pakistanis. Does he do it ?
NO... many players around the world perform it so it is not associated with Pakistani players only ... Yes Amla also performed sajda ...
3) Why don't the bowlers do it after taking five wickets. I think we will see if one bowler does it , the rest will sure to follow.
Amir, Junaid , Wahab , now Yasir are few examples who perform sajda after taking 5fers ...
4) Really does it need to be done right there and then. Isn't religion a private thing.
i think its more connected with ones inner satisfaction and his or her way of thanking Allah for success .... players of all religion do it in many ways .... sajda , cross, dua, eyes towards sky, both hands pointing upwards, kneeling, and many other ways .... in Muslim world you will find many players performing sajda .....
in the end it glorified the celebrations ... i still remember sialkot stallions players collectively performed many sajdas or hoping sajdas one may called it after winning the t20 cup .... so let it be there