A trope concerning works of fiction, especially modern TV shows with continuous story lines. The basic idea is that, once one character is established as bisexual, other characters will "adopt", and suddenly come out as bisexual, too - or at least experiment with bisexuality. This happens even if the characters seemed well-established to be either straight or gay before.
A prime example for the phenomenon is Six Feet Under (potential spoilers ahead). Just for the sake of definition, let's say, "bisexual" within a story arc means having slept with both at least one man and one woman. Up until Season 3, two characters of the main cast, David and Keith, were gay, the rest was straight. Pretty straightforward. Then suddenly, as a new regular character, Russell, is revealed to have cheated on his girlfriend, Claire, with Olivier, the domino chain starts off: "I might be bi," says Russell.
Olivier had slept with women before, yet now he is having a "boy year." Then, in season 4, Claire suddenly becomes attracted to a female student, who in turn is supposed to be a lesbian, but explicitly mentions to have slept with guys as well. Similarly, Keith, one of the gay characters, reveals that he "love[s] fucking women" and proceeds to do so. Keith is, around the same time, hit on by a guy who is married to a woman and had pretended to be straight before, and so on... (end spoilers)*
Several explanations for the occurrence of this "domino chain" are possible. First, it is possible that writers are simply new to the the theme. While they may know that a bisexual character doesn't need another bisexual character to allow for a romantic story arc, they may simply be used to straight and gay characters needing a love interest of the same sexuality (if it's not a hopeless love story). Second, writers who introduce the theme of bisexuality may want to explore it deeper, or get caught up in playing with characters' sexual ambivalence.
But the most convincing reason arguably seems that it's just a matter of convenience. Conveniently, a higher number of bi characters makes the number of possible affairs or romantic links within the existing cast explode. Serials rely on a steady set of characters, and extending the regular cast is expensive and creatively risky. If writers would introduce the topic through nothing but a single new character (or just one established character discovering their bisexuality), it may seem half-assed and contrary to the ambition of exploring the theme in depth. Thus, more than one character will have to at least "turn bi-curious" as a matter of writer convenience.
* More characters were sooner or later revealed to have slept with both male and female partners. Just by the way, this was, in my opinion, the time when Six Feet Under became little more than an extremely well-filmed and fantastically acted soap opera.