You have to look at the whole picture. What is happening to volume prior to the reversal? How long has the trend been going for? Is there any divergence between price and momentum before the reversal commenced? Have there been any bad reports by the company or any bad new stories? What is happening to the market as a whole?
When a reversal happens on heavy volume it may indicate that sentiment has changed, the bulls have been exhausted (no more buyers in the market), and the bears are taking charge (sellers starting to take profits). This effect may cause panic in other security holders and cause them to sell. Then you get other holder's stop losses being triggered and shorters entering the market. If this continues it can cause panic and fear throughout security holders and cause the downtrend to accelerate. Think of it as an uptrend climbing up stairs and a downtrend falling off the cliff at the top. Fear tends to be a stronger emotion than greed.
Something else to look out for just prior to the reversal is intraday highs not holding up by the close. If you look at a candlestick chart this is illustrated by long wicks on the up side with small real bodies. This may indicate that the bulls are trying to push the price higher during the day but the bears push it back down by the close. This is another indication that momentum in the uptrend may be drying up, especially if it occurs over a few consecutive days. If you get this behaviour followed by a drop in price on large volume, it may be the start of a steep reversal.