Here's how Valve described that file earlier today:
The netspike file has long existed in the Source engine as a way to track which entities were causing large spikes of network traffic. There's a convar you can use to configure a threshold when it will get dumped. (I think by default it's off.) Recently there was a change where, if a client was dropped to the infamous "snapshot overflow" error, it would re-run the network packing code for that client, forcing the netspike file to be dumped, to help figure out what entities are the cause of the problem. That could definitely cause CPU spikes. I didn't realize we had shipped with that debugging mode on. We'll make it configurable by convar for the next release.
In the meantime, each time it dumps a network trace, it means you dropped a client. It needed to send them a full snapshot of the game state, but the snapshot was too big, so it had to drop the client. You might want to see which entities are responsible for so much traffic, because you are dropping players.
If there's something particular to your server, this trace will help you tune it. If you find a problem you think is global to the game, please report it here.