I have read that linear bounded automaton is a Non deterministic Turing machine. Why is it so?
The definition of LBA (for example given in Wikipedia) is a non-deterministic Turing machine which uses linear space. So an LBA is a (space-restricted) non-deterministic Turing machine by definition. There could be other, equivalent definitions.
If we are saying non deterministic Turing machine and LBA are same then its wrong. If we say non deterministic Turing machine with limited space then we can call it as LBA. LBA has space boundaries where as Turing machine don't have.