Sounds odd. Perhaps something had happened beforehand, or they're striking in a different sector than what you do. You should really look into what is going on. Break your anxiety and ask.
The point of a picket line is not to physically stop you, though that might happen in extreme cases, but rather to talk to you and have you join the picket line instead of continuing being abused by your employer.
The reason that it should not usually resort to physically stopping workers is that it's likely illegal, but also that there are other more efficient and less dangerous ways to stop the work from being done. Like coordinating with suppliers or distribution. It doesn't matter if you stand at your workplace if your input never arrives and your output goes nowhere. If that's possible, it's only good that you get paid for doing nothing, so it's double as effective.