Too many sick people + Too few beds + Lots of elective surgery is the simple answer. But sometimes the wards stop admitting patients (because it is near the end of their shift and they don't want to do the paperwork) - and then it is up to a Bed Manager with teeth to get things moving. Of course, sometimes the ward doesn't want to admit anyone because they haven't got the right 'staff balance' (i.e. enough competent nurses). It doesn't matter if A&E gets dangerously overcrowded - as long as the wards stay 'safe', it's almost expected that bad things happen in A&E.