Crypto is really unpredictable. While reaping profits of hundreds of percent, the section withstands now and will continue getting dozens of billions of dollars erased flat out in the future. When Bitcoin loses its value against the US dollar Altcoins usually go through the same process. Simple math shows that even holding a part of the portfolio in Altcoins, such as Ethereum and Litecoin, is usually not enough to avoid getting a big chunk of the portfolio’s USD worth wiped out following a Bitcoin dump.
For example, there are cryptocurrencies, such as Litecoin, with the same goal as Bitcoin. In this case, it would be a good idea to compare its market capitalization with Bitcoin. This site ranks coins by market cap. Always be sure to check there when evaluating a new coin. If you notice a large shift in market cap on a certain date, it may be worth it to check for any news that day to see what may have caused it.
Always pay attention to Bitcoin. Most altcoins (every cryptocurrency except Bitcoin) are pegged more closely to Bitcoin than Asian currencies were to the USD during the Asian Financial Crisis. If Bitcoin price pump drastically, altcoins price can go down as people try to exit altcoins to ride the BTC profits; inversely, if Bitcoin prices dump drastically, altcoin prices can go down, too, as people exit altcoins to exchange back into fiat. The best times for altcoin growth appear when Bitcoin shows organic growth or decline, or remains stagnant in price.