The VC market is super hot right now - VC firms are being flooded with cash, and they need to deploy it. Companies are raising massive amounts of money at unprecedented valuations. A large chunk of that money is going to hire developers. The recruiting industry is seeing a huge surge in hiring in the past couple of months (source: I run a technical recruiting company (1)).
Because demand and cash available to hire developers has increased significantly in the past few months, but the supply is basically fixed, companies are offering higher salaries to attract talent. They have the cash for it.
Keep in mind that new money raised from investors will raise the value of your stock options, but rarely translates to salary increases for existing employees.
What a time to be a software engineer.