Is it possible? Yes. Shifts in personality, desires, etc. can happen for a number of reasons. People have had head injuries and had shifts in sexuality, including a heavily increased/decreased sex drive or shift in orientation, as a result. Sexual traumas, particularly on children, can have lasting effects on their sexuality and sexual development both before and after adolescents, possibly (likely even) including homosexual behaviors where it may not have happened before… though that’s obviously not possible to know for sure. And to a lesser extent, situations like long term isolation, desperation, abuse, or social pressure can make people more likely to act in ways that wouldn’t have before as well.

Is any of that the norm or common for homosexuals? No. First, none of those situations is likely to result in a mentally stable, well adjusted adult that simply has attractions to the same sex. Those shifts in sexuality are likely to be just one of many symptoms of those traumas, most of the rest of which will be notably aberrant behaviors or traits. Second, the majority of people who have same sex attraction, whether exclusively or in addition to opposite sex attraction, have similar stories about being interested in the same sex from the time they first began to have any such attractions. Whether there is some genetic, developmental and/or environmental component that differentiates or influences ones sexual interest is not really clear. What is clear is that those interests seem to be largely set by the time they begin to manifest and be expressed in adolescence.