If, as part quantitative easing, a central bank buys government bonds, it might affect prices of other assets positively via arbitrage and the so-called portfolio rebalancing effect. There are obviously several factors at play, but I focus on that one.
If there is more demand for government bonds via QE, bond prices increase, and their yields fall. Assume you have a portfolio of bonds and stocks, QE will then lower the expected yield of bonds relative to stocks. So you may want to sell some of your bonds (at the new higher price, too) and reinvest the money into stocks, that is, you rebalance your portfolio.
If everybody does that, prices of stocks can rise significantly, and largely independently of how stocks are expected to perform, as its about the relative return (with respect to bonds), not about the absolute return. The same applies to prices of other assets, as investors usually have many more asset classes in their portfolio than just government bonds and stocks. Risk management policies or requirements can dampen the effect for certain institutions (for example insurers).
The effect can be exacerbated if institutions take advantage of low interest rates and take out loans to invest in existing financial assets, rather than lending forward to businesses for investment into the real economy as intended by the central bank.