It's usually not a (fully) random order, but rather the least-available (from other peers) parts get prioritised, to ensure maximum overall throughput. e.g. if

It's usually not a (fully) random order, but rather the least-available (from other peers) parts get prioritised, to ensure maximum overall throughput. e.g. if 10 clients have the first part, but only 2 have the second part, it will prefer to download the second part first if possible. But as the other replies say, your client can request any part it wishes. A few clients doing this won't have a significant effect on the overall throughput. Even if most clients were doing this, it would mostly only result in slightly lower download speeds overall. The exception is for torrents with very few seeders, where this can have a much worse effect, but you are unlikely to be able to stream those sequentially at a reasonable speed anyway.