This typically occurs for attributes with the data type of table. When the data type is table, and the box, Allow only one per record, is marked the attribute will show as a dropdown. When the data type is table, and the box, Allow only one per record, is not marked the attribute will show as a checkbox. Likely, the box may have been marked at one point, and then attribute used on a form, displaying the dropdown. Then, the box was not marked for Allow only one per record, and a new form created with this attribute so the attribute shows as checkboxes. 

To resolve, you'll want to edit the attribute within The Raiser's edge and either mark or unmark the Allow only one per record notation.
How to make attributes checkboxes or dropdowns

After the notation is confirmed in Raiser's Edge:
  1. Navigate to Administration>Code Tables
  2. Click Refresh code tables
  3. Edit the forms in NetCommunity which don't have the desired format
  4. Unmark the Attribute in question
  5. Click Save
  6. Edit the form again, mark the attribute
  7. Click Save
This will prompt the data to refresh.