Big, well-established coins like ETH and BTC only go up over time, but that's because there is more and more mining power joining the pool. If people were leaving the pool, the pool's total hash rate can go down, and when it goes down, the time between blocks will get too large, and then the code will respond by lowering the hash rate. The lower hash rate raises mining profitability, enticing miners to return.

If you look at WTM, the third column, labelled Difficulty/Nethash, has three figures for each coin. From top to bottom, they are:

1. Current difficulty.
2. Total network hashrate.
3. Change in percent (probably relative to 24 hours ago; I'm not certain whether this is change in hash or change in difficulty.)

Scroll down the list, and you'll find that there are a couple of coins who have had their nethash go way down; if the list is sorted by current profitability, you'll often find that some of the top coins (other than ETH and ETC) have recently had their hash or difficulty go down significantly.