In summary, what I could find so far, it seems only one of the four bus burnings this year was explicitly claimed
Nov 7 -- Newtownabbey; four men (at least armed with one hammer and one Molotov cocktail) first evict the passengers, force the driver to drive off some way to a mall where they finally set the bus on fire; attack seemingly not claimed, but burned bus phographed/located next to a "scrap NI Protocol" sign.
Nov 1 -- Newtownards; two men (one armed wtih a gun) muttering something about the NI protocol evict the driver at a station [there were no passengers], douse the bus in petrol and set it on fire; claimed by PAF, PSNI suspects a UVF splinter group; date seemingly related to a deadline given by Unionists to scrap the NI Protocol.
April 7 -- Shankill road in West Belfast; during a week of rioting, a bus (apparently first hijacked and driven there) is pelted with Molotov cocktails near a "peace wall"; PSNI ultimately concluded no paramilitaries were involved in that bus burning. The protests were at least in part aimed at PSNI after being seen as going soft on gatherings at IRA events during the pandemic; numerous Molotovs thrown at police vehicles as well. A month before those riots, the Loyalist Communities Council declared that they would no longer participate in the Belfast Agreement (over the NI Protocol).
Various press articles report a 4th bus torching happened this year, but I couldn't find when/where this 4th attack happened.
Among the many barely informative sources covering this, there is one (The Belfast Telegraph--which seems somewhat aligned with unionism) that says that one of the recent attacks was claimed by...
The ‘Protestant Action Force’ said it was responsible and carried out the act to mark the passing of a DUP deadline at the start of the month for resolving the NI Protocol issues.
So at least based on that, the anti-Protocol sign left next to one of the busses (pictured in my question-linked Metro article) seems less than a coincidence.
The DUP condemned the attack[s] though. Additionally,
Stormont’s Infrastructure Minister Nichola Mallon [SDLP], who is responsible for transport services in the region, said the two masked men who carried out the [Nov 1] attack “muttered something about the Protocol” as they held the driver at gunpoint.
According to Wikipedia, PAF might not be a single organization though.
On Nov 4, The Times cited the PSNI saying that a splinter group from UVF might be responsible for the Nov 1 torching.
Interestingly, according to the Irish Examiner, a couple of days after the penultimate bus attack [Nov 3]
the Progressive Unionist Party (PUP) said it could no longer support the 1998 Good Friday/Belfast agreement because in its opinion the Irish Sea border had undermined its central plank – the principle of consent.
(Somewhat oddly, this article doesn't have any direct quotes from PUP, but has a lot of quoted from the DUP commenting on what PUP is said to have said. There are however more extensive interviews with PUP leadership, which seem to mostly confirm that. However the PUP leadership stressed that opposition to the Protocol should be “peaceful and democratic”.)
Interestingly however, in the April events, which also saw a bus burned, the police ultimately said no paramilitaries were involved. (I'm unsure when the 4th bus was burned.)
On the other hand, the Loyalist paramilitaries themselves (or at least a group called the Loyalist Communities Council [LCC], claiming to represent the Ulster Volunteer Force, the Ulster Defence Association, and the Red Hand Commando) had announced they were withdrawing their support for the Belfast agreement in March, until the NI Protocol
is "amended to ensure unfettered access for goods, services, and citizens throughout the United Kingdom." The LCC added however that opposition to the protocol should remain “peaceful and democratic”. What this seems to have meant in practice is that they have stopped cooperating with the police, e.g. on defusing riots, at least.
On that angle, there's an interesting ITV documentary on what you could easily call in other contexts the new generation of radicalized youths. Most of the teenagers interviewed justify the riots with vague reasoning like "what had to be done had to be done", but one is more explicit and says that until violence erupts the media just ignores you, so this--he says--is the only way to be heard.