This was weird but I was able to figure it out, so here is some PSA to all: from time to time as I backed up my car onto my driveway I would notice my Model 3 would automatically shift into park. After calling support and some troubleshooting this is what I was able to find out and reproduce: if you have your seatbelt unbuckled AND you take weight off the driver seat (which apparently sometimes I do by supporting/pressing my right foot on the foot rest) then Model 3 will shift to park as a safety measure (kind of like it thinks there is no one or no adult and the car is rolling).