"Trap" when used to describe transwomen or genderfluid people becomes inherently problematic due both to the awful reality of how often it plays into the imaginations of people who either kill trans people themselves or otherwise support oppressive legislation built around "trans/gay panic" ideas. It also is inherently possessive of the person's experience because it seeks to establish their purpose as sexual tricksters entirely related to them, the cissexual heterosexual (usually male in media though there are enough TERF lesbians to make it a thing in flesh-space too.) It does not consider the trans person as a
person with their own needs. They're simply there to elicit an "ewwww gay" response at the reveal, intrinsically directed at the trans person but also the interested party unless they do something (such as show their disgust) to prove their sexuality, something that's pretty screwed up in itself.
And there are plenty of people who enthusiastically endorse these views, even if they don't advocate for full on murder themselves (and that is an
if). Whenever I have seen a "trap" scene that can be publicly commented upon there are several posters remarking how they would "punch the fa---t" or otherwise initiate hostilities if they were caught in that situation. For trans people this brings up legitimate fears that are placed upon them by society and friends who've been lost. But the fact that the memes behind "trap" depictions are hurtful (emotionally, physically, and in social influence) usually doesn't stop cis people from gleefully using it for their own entertainment.
As for Akiba Trip, briefly looking at the response I can see why people would be upset with them.
The most important thing to note here is that we're not actually using "trap" as a transphobic slur at all. The character who's being called a "trap" is being accused of pretending to be a girl on the internet in order to curry favor with her male co-posters. She is NOT being accused of being a transgendered woman -- the idea is one of deception, not trans*.
This is important to note, because the word itself is not ALWAYS used as a slur. In fact, in common internet parlance (especially on 4chan), it's used for exactly the purpose we've used it: to suggest that someone is not actually female, but pretends to be (and is quite convincing about it). Thus, a "trap": someone who fools men into thinking he is a she, trapping them into flirtatious behavior, including money and favors.
This ... is both transphobic and misogynistic. People opposing legal rights for trans people also cast their worries as only targeting "fake trans people" in an attempt to avoid accusations of discrimination. And the whole idea that one pretends to be a woman for favors is the sort of sexist nonsense that, rather than targeting people for their acts of manipulation, decides to make it a matter of gender and women inherently obtaining advantages from men who apparently throw things at random people over the internet. Implicitly with the promise of eventual sexual intercourse, since apparently giving people stuff from outside your sexual orientation is out of the question. Maybe some men, convinced of this same ideology, attempt to use it against others who are absurd enough to send strangers things and then don't accept the responsibility of their actions, but they can be assholes without people conjuring the fear of "fake women" that is also a dangerous meme in society.