Why was my stop-loss executed below the trigger price?

This happens with a stop-loss market (SL-M) order, and it’s expected behaviour, not an error. Once the trigger price is hit, an SL-M order converts into a plain market order and executes at the next available price in the order book, which can be lower than the trigger, especially in a fast-falling or thinly traded stock where there isn’t a buyer waiting exactly at your trigger level. The gap between the trigger price and the actual execution price is a form of slippage, and it tends to be larger during high volatility, low liquidity, or when a stock is falling quickly through several price levels at once. If you want to avoid execution below a certain floor, a stop-loss limit (SL-L) order sets that floor explicitly, with the trade-off that the order might not execute at all if the price falls too fast past your limit. You check whether you used SL-M or SL-L before assuming something went wrong, since SL-M by design prioritises exit over exact price.