Table:
text: 2007–08 Leeds United A.F.C. season
In
In
Name_ Country Type_ Moving_from Transfer_window Ends Transfer_fee Source_
Ankergren DEN Transfer Brøndby Summer 2010 Undisclosed Leeds United
Constantine ENG Free Agent Port Vale Summer 2009 Free Leeds United
Weston ENG Free Agent Swindon Town Summer 2009 Free Leeds United
Prutton ENG Free Agent Southampton Summer 2009 Free Leeds United
Thompson ENG Free Agent Celtic Summer 2008 Free Leeds United
Hughes ENG Transfer Norwich City Summer 2009 Undisclosed Leeds United
Martin SCO Transfer Motherwell Summer 2009 Undisclosed Leeds United
Huntington ENG Transfer Newcastle United Summer 2009 £150k Leeds United
da Costa POR Free Agent Ionikos Summer 2008 Free Leeds United

Question: What is the reason for Port Vale's move from Leeds United AFC for the 2007/08 season?