I was able to find the referenced account and see how an order to short USWS (even if inadvertent) created a day trade when sold the same day. That, combined with day trades of two other stocks, added to 3 day trades today. If an account executes more than 3 day trades in a 5 day window the account would get flagged as Patern Day Trader (PDT). As such, the account would not be permitted to trade at all if the equity drops below $25,000. The restriction keeping one from executing a forth trade is to prevent the account from being further restricted to no trades.
These regulations are imposed by the SEC and FINRA. All brokers, including Alpaca, are required to enforce them and Alpaca is audited to ensure compliance.
Alpaca encourages anyone with less than $25,000 equity to refrain from any day trading for reasons exactly as was this case. The 3 day trade allowances are meant to be used in ‘emergencies’ and generally not to be part of one’s regular trading strategy.
You will be able to exit positions tomorrow, but also note you will not be able to place any day trades for 5 days since there have already been 3 today.
While this may seem an inconvenience, it is part of the regulatory framework which all traders must work within.