This happens when you use objects from the Control Toolbox toolbar.
Office considers those objects to be in the same security
classification as macros, because they're ActiveX controls. If the
recipient has the macro security level set to High, the document will
open in Design Mode and the controls will be disabled. The same thing
will happen if their security level is set to Medium and the recipient
clicks the Disable button in the warning message when opening the
document.
Since you can't control the setting of anyone else's security level,
the only choice is not to use those ActiveX controls. Use only form
fields from the Forms toolbar, and then protect the document for
forms. See this series of articles for a tutorial:
http://www.computorcompanion.com/LPMArticle.asp?ID=22
http://www.computorcompanion.com/LPMArticle.asp?ID=46
http://www.computorcompanion.com/LPMArticle.asp?ID=119
http://www.computorcompanion.com/LPMArticle.asp?ID=127
http://www.computorcompanion.com/LPMArticle.asp?ID=136
Also, here's an explanation of ActiveX controls:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnword2k2/html/odc_activeX.asp