I will admit that I really don't like HK alternative media (i.e. most of those Hong Kong Facebook pages and Youtube channels) so take this with a grain of salt.
TL,DR: This is the result of global trends of turning conservative, alongside unique problems in Hong Kong media.
Despite being seen as a westernized city, Hong Kong's culture would still be considered conservative to those countries. Young people are slowly changing things but it is limited in some areas - something like racism is improving slower than sexism, for example. Political awareness is also low due to historical reasons, even the more progressive aren't above great man theory of politics. Also doesn't help that,
in Hong Kong, left is understood to be related to communist (左仔=leftists=CCP) while right is aligned to traditional western values (a Cold War-era designation). That explains media like Apple Daily being right leaning even before the protests.
As such, the initial support for Trump should be obvious. The protests and subsequent US reaction happened during Trump's presidency. Most people don't understand more than that - even for those who are well learned in HK/Chinese politics, US politics is something else altogether - and become a core part of identity as the protests and traditional pro-democracy groups got crushed.
In Hong Kong this is compounded by the destruction of traditional pro-democracy media outlets, meaning that outside of pro-establishment voices, people have little choice other than alternative media that rely on donations.
As I understand it, election fraud thing isn't a majority position talked about but those who disagree become silent for one reason or another, allowing the radical to use these theories to fight for the (very limited) screen time and money that Hong Kongers have for those alternative media.
That's about why I think you see so many of those conspiracy theories on HK social media.
ps. I may sound like a broken record at this point, but wherever you are from, it would be a mistake to think it's just Hong Kong. I can see a lot of countries needing to rethink their political stances and relationship with (new/old) media going forward. The meteoric rise of reliance of alternative media during COVID happen alongside the move to the right, this is not just happening in Hong Kong but in many other countries.
Quoted from /hong kong
https://www.reddit.com/r/HongKong/s/Qtbh4e1Oj4