After listening to LiveATC associated with this incident, some observations:
Female FO was PF for the landing, which implies PF/PM swap after landing.
At times I had a hard time understanding the local controller due to rapidity of his speech and allowing speech to trail off at end of each transmission. Pilots seemed to understand his instructions with no difficulty.
On landing rollout, local controller says “United ah 384, you got [at?] Kilo? Female FO quickly responds, “United 384, turning left on Kilo.” Local controller immediately says, “United 384, hold short Four Left.” Response is “Hold short of Four Left, United (long pause, mic unkeyed and rekeyed) 384.” About 25 seconds later, local controller says, “United 384, continue to cross Eight Left, contact ground, point niner.”
From ADS-B data, UAL384 had a ground speed of 35 kts turning onto Kilo, 24 kts as it crossed the midpoint of 4R/4L on Kilo, and 15 kts crossing the 4L/8L intersection. There was no stop.
My initial assessment is that, upon hearing “you got [at?] Kilo,” UAL384 took this query as instruction to use Kilo, and assumed that if the local controller approved use of Kilo, approval to cross 4L/8L was either included or would follow shortly. I think the local controller was asking UAL384 if they were able (groundspeed slow enough) to turn at Kilo. By the time the UAL384 crew read back the hold short of 4L instruction, they knew they were already over the hold bar and there was no way they could safely avoid entering 4L/8L with their groundspeed.