In altcoins, why is it that sometimes the volume increases when the price stays relatively stable or even goes gown?
Volume is simply the amount of trades in a certain period of time. An increase in volume usually correlates with a change in price, however this does not necessarily imply an increase. Your observation mainly as a price increase is likely due to the current bullish market. A high volume can represent a lot of buyers demanding a coin like it can represent a lot of people selling. A sideway market could be seen simply as more people buy and selling thus increasing the volume without increasing the price.