Since market capitalization is based on the volume of outstanding shares and the price per share, a change in either one will cause a market cap to rise or drop. The most obvious way a company’s market capitalization can increase is to increase their stock’s value (i.e. for their price per share to rise). This essentially means a company meets or exceeds performance expectations and is rewarded by having investors want to buy shares of their company, which in turn drives up the price of each share.
A second way is for a company to issue more shares. This is typically done because the company is looking to raise capital. At first, investors may see a slight dilution in share price as more shares are put on the market, but the market cap would still increase unless the share price took a big hit for another reason.