Hi Sunanda, thanks for the question.

There can be my different reasons for this to happen.  This could include girdling roots, verticillium wilt, physical damage, to roots due to construction, environmental stress, et al. Unfortunately, it would be difficult to determine the cause from the pictures.  Probably the best way to get a diagnosis would be to get a certified arborist to assess the situation.  You can find one here:

Find an Arborist (treesaregood.org)

I hope this is helpful.