Good example of outfits sending signals.

Slacks and a button-up probably made you look like management just out picking up slack, and people subconsciously left you alone - "oh, they're probably really busy with management stuff and also had to go shelve books.. I'll leave them alone"

Casual clothes in contrast would have signalled "worker" - "I'll just go ask that person, it's not like they're doing anything important, they're just shelving books!"