Why did you rebuild the publication and subscriptions after a simple subscriber reboot? The transactions would of just built up on the publication DB and then synched up once the subscriber came back online. regardless to answer your question...

Syncing up the subscriber database uses Snapshots which will create locking on the publication DB as you noticed. The best thing to do is not set to Immediate initialization and either do it manually during off hours or simply set a schedule to do it during non-peak hours.