[hooper]

Debris orbits like anything else. Regardless of the orbits of the colliding objects, the orbit of any debris is only really constrained to intersect the point of collision. For some debris, the point of collision could be the lowest point in its new orbit, meaning it could take longer to reenter and could collide with other stuff higher up until then. On the other extreme, some debris could essentially fall straight down to earth immediately.

[KyleBerezin]

No collision of 2 surfboards is going to loft debris high enough to be in a stable orbit. If the common point of orbit is 500km, all of that debris will de-orbit. The only propellent on those things is krypton, so if they collide it is only going to be mechanical effects on the debris. In my search I have not found a single reputable source claiming starlink could contribute to kessler syndrome, only clickbait articles.

[hooper]

The debris orbits from an initial collision don't have to be stable to allow a chain reaction that produces debris in more stable orbits. Debris from even a single low altitude collision/disintegration could collide/disintegrate later at a higher altitude, producing debris with a higher periapsis than any of the original bodies.

There are many factors in whether this matters practically, so I'm not passing judgement on that. None of this is specific to Starlink.