Looked at altcoins and they've started rallying pretty hard all at once, whats the reason for this? Is it because of Bitcoin going sideways for a decent while or some other reason?
It is caused by bitcoin moving sideways for a while, which led to a big pump on bitcoin's price.
When bitcoin gets pumped, altcoins historically have also gotten a lot of increases in price and market cap. So that is the reason for the rally. I do't think the bear market is fully over, so take your profits wisely now that the price is a bit higher.
Altcoin prices will continue to depend on bitcoin prices since it's the face of cryptocurrencies and #1 in market cap. So if bitcoin is in a bear market or is getting dumped, so will altcoins, and vice versa in a rally.