One, it is incredibly rare for women to set this up because even if they 'win', the long-term detrimental damage to their reputation and tag it creates is too much to recover from. And while yes, you're right about a fair trial, understand that the victims of these crimes barely have any proof to present on the long run. Hence the onus to believe them first, and also why so few of these actually make it to court.
Two, It's not even sexual assault in particular on the initial tweets, straight up direct bodily harm and one of the victims getting their injuries treated. Which effectively means either a) Giggs, with a proven reputation of being a human being with no shame, actually beat up a couple of women, vs. b) There was a woman who hurt herself to get take him down/get a pay day/etc. That's not a hard guess.