Stress
Subbing is infamous because it's a position of incredibly high stress—and incredibly low stakes. When a president gets stressed and all their hair turns gray, it's because they're deciding the fate of nations. When you get stressed, it's because students glued your pants to the seat.
Kids don't often respect subs, and depending on their age they'll push as hard as they can to see what reaction they get from you. If this is your first time subbing, you're in for a treat. Kids will ask for things they would never ask their regular teacher for—"Hey, Ms. Potter always lets us watch Twitch streams while we learn about math and junk. Can we put them on the projector?"
Ten minutes later, everyone's watching HyperPlr721 totally own League of Legends—in Ms. Potters' ninth grade science class.
If things really get out of hand, a full-time teacher or even a school administrator might step in, but that won't look good for you. Subs are basically judged on their classroom management, so if you can't keep order, you also can't keep your job.