Histrionic Personality Disorder is currently defined - in the US 'Diagnostic and Statistical Manual of Mental Disorders' DSM5 - as :
A pervasive pattern of excessive emotionality and attention-seeking, beginning by early adulthood and present in a variety of contexts, as indicated by five (or more) of the following:
The cause - or causes - of HPD are unknown.
Little research has been done to find evidence of what causes histrionic personality disorder. Although direct causes are inconclusive, various theories and studies suggest multiple possible causes, of a neurochemical, genetic, psychoanalytic, or environmental nature."
Source : Wikipedia