My staircase joint has become misalign due to internal movement - for which I don't know what.

Hi It would be best to get a carpenter to look at it. You can usually repair and replace most parts on a stair case with out replacing the whole staircase.

if your timber members of the house are moving it would sergest 1,a change in the heating in the house ,2 , a change in the construction (have you had any walls taken out) 3 ,damp (if one of the strings are against a out side wall)or i have come across rot in first riser & newel at floor level. if its a new house contact N H B C