Visible crew/equipment: At the drive in theater, Marty says he'll put the boots on when he gets there. The camera pans to the left to follow Marty and Doc as they continue to talk. On the very far left side of the screen, the shadow of the boom mic briefly dips into frame. This happens just before Doc walks over to the DeLorean from his car. (00:16:35)
Visible crew/equipment: In the scene where Marty is chased out of the saloon in 1885 by Buford "Mad Dog" Tannen & his gang, in the shot showing Marty pushing his way between two women, the cameraman can be seen running along with the camera beside him to the left of the screen. (00:32:26)
Visible crew/equipment: During the party scene, towards the end when Marty and Doc are discussing the duel and the fact the train may be late, you can briefly see a PA system center left of frame (bass cab with PA head on top and above it 2 tweeters side by side) it would make sense that a PA system be on set during the scenes so the band and dancers had a soundtrack to mime/dance to. (01:01:29)
Visible crew/equipment: When Marty is with Jennifer in his new truck, as the camera cranes down to where they're talking, you can see a crewmember with a white shirt and some of the camera and crane. (01:46:10)
Visible crew/equipment: In the saloon scene where Doc passes out, a production light with barn doors and stand can be seen in the upper left over Marty's shoulder.
Visible crew/equipment: When Doc's about to send Marty back to 1885 from 1955 at the open air cinema, you can see the shadow of the camera sticking out from the shadow of the building - it moves as the camera tracks Doc over to the car.
Answer: He simply has an absurdly low tolerance for alcohol, and whiskey is not a wise choice if this is the case. It helps set up the joke when Marty asks the bartender, "How many has he had?", and he replies by telling Marty, "Just the one", as we are meant to think Doc has been in the bar all night drinking away his sorrows.
Jazetopher