These statements aren't necessarily contradictory.
In the first case, investors are bearish because they anticipate selling in the future (because all the interested buyers have bought, so all that remains in the short run are people willing to sell and therefore drive down the price).
In the second case, the trend is strengthened because the increase in volume indicates that the price movement interested a lot of traders. The trend could be bullish or bearish.
The statements aren't contradictory because the second case could very well lead to the first case. For example, if an increase in price is coupled with an increase in volume, this could indicate that the positive trend is strengthening (second case). Traders are becoming more interested in the price move, so they buy.
However, once all of the traders who are willing to enter the market long do so, we're in the first case. Investors realize that all of the traders who were interested in buying have bought, so they become bearish because they expect selling to start soon.