It's a bit too soon to be 100% sure on the details, but it looks closer to what we'd call mob / mafia violence.
The perpetrators wearing white shirts were working for the triads, similar to the Mafia in the USA, or the Yakuza in Japan.
See
https://en.wikipedia.org/wiki/Triad_(organized_crime)
“I have strong reason to believe they were gangsters,” said Lam
Cheuk-ting, a pro-democracy lawmaker who was hit on his arms, hands
and face, leaving his mouth with a cut that required 18 stitches to
close. “I don’t think any ordinary citizens have done such
sophisticated, organized attacks on this kind of level.”
This was later confirmed:
Hong Kong police detained six men on Monday in connection with the
attacks. Some came from rural parts of Yuen Long. They ranged in age
from 24 to 54, and their occupations included drivers, hawkers and
renovation workers, senior police official Chan Tin-chu said.
"Some of them have triad backgrounds," he told reporters, referring to
organized crime syndicates that hold sway over certain neighborhoods
in Hong Kong.
They are driven by money, not by politics alone:
“The Hong Kong triad only works for money, not for political
ideology,” Lo said. “They will work for anyone.”
We know what their original political ideology was: the restoration of the Ming empire.
They have their roots in mainland China; the first triad was a
patriotic secret society formed in the 17th century to overthrow the
Qing dynasty, which had been founded by Manchu invaders, and to
restore the Han Chinese Ming dynasty. By the beginning of the 19th
century, the group had disintegrated into gangs operating
independently all over China. Their membership in Hong Kong surged as
refugees fled civil war and political upheaval on the mainland.
It's unclear if they were paid by the government or not, but it is possible.
“Despite their current image, [the triads] are not actually
‘patriotic,’ just opportunistic,” says Vickers, who ran the Criminal
Intelligence Bureau in the colonial administration. He likened the
triads presumed role in furthering China’s national interests to the
“alleged but unproven CIA and mafia cooperation in the 1960’s.”
It's also possible that the corporate leaders in Hong Kong's business district now see the police as ineffective, and might see the triads as an effective option:
In this case, the attackers could possibly have been hired by business
interests who, like the government, want the protests to end, Ong
said.
The Yuen Long region, where the attacks took place, has a long history
of triad activity particularly with 14 K and Wo Shing Wo subgroups,
Vickers said.
In the last couple of weeks, there has been considerable tension
between protesters and local businessmen who are very opposed to the
demonstrators and their activities, Vickers said.
"So the question of who really 'pulled the trigger' remains open," he
said
So in conclusion, I think it falls under mob violence / organized crime.
It does share many similarities with terrorism, but I think we are too quick (maybe rightly) to assume terrorism nowadays because of how much terrorism has happened in the past few years.
