Blue tit chicks abandoned

Hi all, I’ve a pair of blue tits nesting in my bird box, recently they have been hard at work in & out of the nest so I assumed they had chicks.  I noticed this morning that I haven’t seen them today & yesterday so I took the plunge & opened the box. Inside were 9 dead chicks, abandoned clearly.  I don’t know how late I was and how long they had been dead.  Why would this be? Could there have been too many chicks to look after so the parents have given up?