Thinking of my DD's (very well meaning but inexperienced and unsupported) former preschool:
Physical barriers: Poor room layout, with ready access to restricted areas (ie. kitchen with no lock) meant she could get herself into trouble.
Room too open plan - meant she got overwhelmed and would react by running around aimlessly.
Poor outdoor facilities - multi-level concrete grounds meant she required full 1:1 to prevent injury due to her physical issues.
Lack of permanent use of building: Restricted equipment dependant on staff being able to get out and set up, which in turn meant DD1 would find her own physical stimulation 
Environmental factors - sensory issues, noisiness of building, numbers of children
Staffing factors - staff leaving, inconsistency between staff, staff unsure of how to deal with DD1.
education factors - staff unsupported and left to 'get on with it', staff unable to implement suggestions by professionals becuase of assumed knowledge and lack of confidence to say 'I don't know what you are talking about'.