There could me multiple reasions for the status date to change:

• Reviewer comments are added to the database
• The reviewer changes
• Editor checks the database

There could me multiple reasions for the status date to change:

• Reviewer comments are added to the database
• The reviewer changes
• Editor checks the database

See this question: http://www.editage.com/insights/why-does-the-status-date-of-a-submission-change-periodically-though-the-status-remains-unchanged