The obvious answer, which lots of the others are suggesting, is that Side A was at the end - there was nothing left to play. When you turned the tape over to Side B the tape was at the beginning ant it would play.
It's hard to believe anyone would make such an obvious error though.
I've had cassettes that wouldn't play and were “fixed” by using fast forward or rewind. I don't use cassettes now but, really, it was just short of a miracle that they worked at all and produced a listenable sound. I found that over the years I had just about every problem with cassettes that was possible.
Added, I've never come across a cassette played that automatically rewound one side of a cassette back to the beginning. That would be a really pointless, frustrating feature!