It’s a danger to them, to drivers, and to other cyclists (the ones coming down the other direction).
In my experience it’s not a constant problem but it’s not a trivially small number either.
In the absence of a bike lane or wide shoulder, I don't feel that it matters much as it's just an unsafe situation regardless - there's an argument that a large group in a pack is shorter and therefore safer and easier to pass, an argument that having to swing "wide" into the opposing lane is more dangerous, an argument that cyclists in this situation simply should not be passed (in which case IMO they should pull out at the next opportunity, just as a slow motor vehicle would be required to do in many states), and an argument that the situation shouldn't exist to start with.
https://leginfo.legislature.ca.gov/faces/codes_displaySectio.... https://leginfo.legislature.ca.gov/faces/codes_displaySectio....
Any person operating a bicycle upon a roadway at a speed less than the normal speed of traffic moving in the same direction at that time shall ride as close as practicable to the right-hand curb or edge of the roadway except under any of the following situations:
(1) When overtaking and passing another bicycle or vehicle proceeding in the same direction.
(2) When preparing for a left turn at an intersection or into a private road or driveway.
(3) When reasonably necessary to avoid conditions (including, but not limited to, fixed or moving objects, vehicles, bicycles, pedestrians, animals, surface hazards, or substandard width lanes) that make it unsafe to continue along the right-hand curb or edge, subject to the provisions of Section 21656. For purposes of this section, a “substandard width lane” is a lane that is too narrow for a bicycle and a vehicle to travel safely side by side within the lane.
(4) When approaching a place where a right turn is authorized.
(b) Any person operating a bicycle upon a roadway of a highway, which highway carries traffic in one direction only and has two or more marked traffic lanes, may ride as near the left-hand curb or edge of that roadway as practicable.
https://leginfo.legislature.ca.gov/faces/codes_displaySectio....
Each time a cyclist has annoyed me or created an extremely dangerous situation of their own accord, it mostly just came down to poor route selection by cyclists, which represents an infrastructure problem, or exactly one traffic law violation: speeding.
Cyclists riding in packs represent a route selection and availability issue. Large groups of cyclists have to decide between being annoying or being dangerous every time they ride on a road without a bike lane or large shoulder. Whether they're riding two or three abreast (in which case everyone just has to wait for them, the annoying choice), or single file in a line (in which case vehicles have to cross the center line for a very long time to pass safely, the dangerous choice), there is no safe and friendly way for them to coexist on these routes. Now, of course, these cyclists did make the decision to do this - so, the debate here becomes fairly existential. Personally as a cyclist, I avoid these roads and situations, but I'm not convinced that "these people shouldn't be doing this" is a societally beneficial option here.
Cyclists who speed aggressively do really annoy me. Both on footpaths, where they build animosity with pedestrians and create dangerous collisions, and on mountain descents, where they regularly out-ride their sight lines and braking ability.