Hi Elham,
The default product category is picked from the org structure of the user.
PPOSA_BBP > User's Position > Extended Attributes > Product Categories. The 'Default' check box is selected against the default product category (2000 in this case).
If the above doesn't help, please check if the Pdt Cat - 2000 is inherited, if so, then you have to remove it as the default at the top node (where the assignment is done).
Check if any custom coding has been done in the system to force the default to 2000.
Hope this helps 
Cheers,
Yateesh