If you also notice it looks like Pete shat in his pants at Woodstock.
Pete admitted that when he’s on stage he’s totally a maniac and would probably try to kill you because he’s so focused on what he’s doing.
I noticed that a few years ago, when the deluxe edition came out. I know The Who supposedly was fed LSD-laced drinks backstage and were “flat-out tripping against their will” for their set. It wouldn’t surprise me if Pete indeed soiled himself that morning.
Maybe he was just possessed.
AC/DC admits that in an interview.