Microsoft Dynamics CRM Update Rollup 12 introduces a change that limits the amount of open editable Microsoft Dynamics CRM forms to five.
Microsoft Dynamics CRM has a unified user interface experience between its Web Client and CRM Outlook Client. Web Forms can sometimes contain quite a bit of content, data, and code. To ensure you, as the user, have a great experience in our CRM Outlook Client, we have designed a feature that will optimize how you open Microsoft Dynamics CRM forms. When you open more than five Microsoft Dynamics CRM forms, we will be begin to mark as read-only, the oldest forms that have no changes. The form will be designated as read-only by making it gray. If you decide to focus on a read-only form, it will reload into its edit mode. At any time you can many have five forms in Edit mode.
This was introduced to reduce the stress on virtual memory that has caused performance and Out Of Memory issues.