As mentioned here, all behaviour is communication, and challenging behaviour is unlikely to ‘come out of the blue’.  The Challenging Behaviour Foundation outlines the different stages in finding the reasons for challenging behaviours.  The information sheet discusses:

• Why does it happen?
  • Health
  • Change
• What is the purpose (function) of the behaviour?
  • Social Attention
  • Tangibles
  • Escape
  • Sensory
• Setting Events
• Identifying Triggers
• Different Stages of Behaviour
  • Proactive
  • Active
  • Reactive
  • Post Reactive
• Functional Assessment