I think it was probably a case of Swiss cheese:
1. Low morale amongst players
2. Poor goalkeeping options
3. Poor striker options
4. Confused tactics (possibly due to the above), which got more confusing the more desperate he became
5. High expectations
6. Lack of Championship experience
But...most importantly
7. He didn't look like he could motivate a squad that had been through a horrendous PL campaign or the fans for that matter
7. Plus he was fucking useless