Not sure about the US but this is more or less what happens in Japan:

Brokerages are obligated to flag and report suspicious orders or trades to the [equivalent of] SEC.

If necessary, the SEC then asks the brokerage for more information and investigates it.

The definitions of "suspicious" can be somewhat arbitrary, but those >1000% returns would definitely be considered suspicious. Two people at the same workplace getting abnormally high returns would also be considered a red flag.

Yes! That's the question! Was it that they were too successful trading, or was it their employer (or a co-worker) reporting on their side-business to the SEC.