The Americans release the details behind the extradition:
https://imgur.com/a/8iRToyc
Turns out that she (specifically) is accused of multiple counts of international bank fraud. Even knowing this was the case, it sounds like she got over confident and assumed that travelling through Canada (instead of the US) would keep her from their reach.
Back in 2016, Huawei itself was under investigation for other things in America and its senior officials there were issued a grand jury subpoena - and adjusted their travel plans afterwards.
Apparently HSBC might be part of this as well (Hong Kong bank of Canada) - they were the first to also notice some unusual activity.