Date: 202102
Local time of day: 0001-0600
Location: ZZZ.Tower, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ZZZ; Tower ZZZ
Reporter organization: Government
Reporter function: Local; Ground
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Distraction
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Taxiway
Detector: Person Air Traffic Control
When detected: Taxi
Result: General None Reported / Taken
Contributing factors: Environment - Non Weather Related; Human Factors
Primary problem: Human Factors

Narrative: This was at the very end of the swing shift; where I sent down the other two controllers for COVID clean-up protocol.  There were four arrivals - 2 pairs - so I paused on cleaning to work them down and to their ramps.   Aircraft X landed Runway XY Right; with an aircraft rolling out on Runway XY Left; with another aircraft on Taxiway T.  It looked like Aircraft X would exit T as well.  I gave the Runway XY Left arrival turn left on D join A to the ramp this frequency.  I then gave the first aircraft at Taxiway T cross Runway XY Left; turn left to D while crossing and join A to the Ramp this frequency.  I gave them both A because they were parking at the D Ramp.  Then I gave Aircraft X cross Runway 28 Left; turn left on D; then left on B; taxi to the Ramp via B; H; A; this frequency until switching over to Ramp Control.  I gave Aircraft X; B due to parking at the B Ramp; and to not get stuck behind the other aircraft should they not have marshallers to enter the ramp.  Once this was all communicated; I resumed cleaning and the first mid shifter came upstairs.  I noticed at this time that Aircraft X had joined Taxiway A behind the other aircraft and watched to make sure the other aircraft were able to get into their ramp space.  They were and I was satisfied to file a report.  The mid-shifter plugged in and I began the briefing with mid shift closures; and then followed the check lists to cover CIC; Local; Ground; and Clearance Delivery.  As I was covering the Ground traffic; I noticed Aircraft X was now on B between G and H as they should have been originally.  I am not sure why they corrected when the other two aircraft were out of their way.  I completed the briefing and left.  I have no recommendations for this event.  I would however; like to hypothesize why I believe this occurred.  I think that the preceding 2 aircraft crossing T to D to A; created an expectation bias with Aircraft X to follow them without having been told to.  I believe they were finishing up their after landing checklist and confirmed between them that they were to have been on B; and joined B at F1 hoping that the diagonal versus 90 degree turns would be subtle enough that ATC would not notice.  I further believe that we are all running around 10% acuity with the pandemic and all of the circus atmosphere of the recent political debacles.  This would explain the simple mistake and due to the reduced traffic there was no harm to the operation.  Finally; there is a chance that with nighttime and possible lack of familiarity; the crew was confused by the taxiway lights at the junction of Taxiway's A/B/D.




Synopsis: Tower Controller reported a taxiway incursion due to the aircraft flight crew not following clearance instructions.