That is called a startle reaction. It is not a phobia. Severe startle reactions are often related to childhood trauma. Its is most defiantly not homophobia
That condition is called Paranoia.
paranoia
Having the fear of being stood behind is a form of claustrophobia. Claustrophobia is a broad term for the fear of confined spaces.
Fear of Asians is called sinophobia.
If fear of gays is homophobia, then fear of straight people would be heterophobia.
Gerontophobia - fear of old people.
Stairo-phobia
Socianthphobia
Anthropophobia.
Sociophobia.
Fear of people is called anthropophobia.
Nomatophobia
The phobia of things behind you has not been officially identified. This is not to say that there are not other phobias that include having a fear of things behind you.
because many people have a fear of themselves