Renaming field

Hi All

I have question about one strange behavior of Sales Header (Sales Order). Here is the thing:

I should do simple thing: When “Sell to Cust. No.“ is entered in a Sales Order and a comment exists for this customer, a message should appear: 'Cust comment exist'

I add only this code in a OnValidate trigger of 'Sell-to-Customer No." in Sales Header table:

  IF "Document Type" = "Document Type"::Order THEN BEGIN

     CommentRec.SETRANGE("Table Name",CommentRec."Table Name"::Customer);

     CommentRec.SETRANGE("No.","Sell-to Customer No.");

     IF FINDFIRST THEN 

        MESSAGE(TEXT001);  

  END;

(There is no other code in this trigger (only standard and this above))

After adding of this code , when I try to create new Sales Order, and filling  'Sell-to-Customer No."  field appears this NAV msg:

             "Do You like to rename the record?"

I don't get why this msg appears, what is reason for that? When I remove custom code (that one above), that msg doesn't appear when I choose 'Sell-to-Customer No." 

and 'Sell-to-Customer No."  field is not set as a part of primary key, so how I am renaming the record with choosing customer? I am a little confused.

Can anybody tell me the reason for this?

And what  to be done for avoiding this?

Thanks in advance