ETH and other altcoins generally follow BTC. When BTC is in a bear market, they dump harder. When BTC is in a bull market, they pump harder, although they often stagnate, realizing their exponential gains over very short periods of time.

BTC similarly follows the broader risk asset markets (like stocks and gold) the same way. Now that risk assets are experiencing strong bull trends, this reinforces bullishness in the crypto markets.

The combination of fiscal and monetary stimulus has significantly bolstered all these markets. In an attempt to offset the effects of demand shock and unemployment, the US government alone injected something like $5-6 trillion into the economy, and they did it very unevenly. Lots of corporations and middle class people instantly had their costs evaporate through loan deferments and forgiveness, not to mention shoveling unprecedented amounts of money at the unemployed, who have been making more money on unemployment than their former jobs.

In other words, there was a shitload of new disposable capital waiting to be spent or invested into the markets. Some of that definitely ended up in the BTC markets too, however small.