Maybe device/client-dependent? A narrow screen would be a single column, a wider version might be hardcoded to have 3 or 4-button rows?
A reminder that once we make things user/developer-configurable (actions in this case), trivial things can become complex.
eg.
The design of actions is not an easy topic.
First, I actually think actions should come with its own icon, but we couldn't get around to define its syntax as we were rushing to release the first ios/Android which supports the test case. Once you have icon with text below them, the user might mentally accept mulit-column action buttons easier? That is a direct answer to your question.
Looking forward, I expect that if actions are often used in the future, there might be default actions with default parameters. Imagine an Acron-like service. Every time when you spend some money, you save the change in a piggy bank account, which provides an action "save" that can stack on spending. Let's assume the user spends from a multi-sig wallet. The action is rather simple - if it pops out a form, it might be filled by default and automatically so. Therefore you might expect an iconified action that can be easily attached to other actions. See the drawing I made and observe the "Save" iconified action which is used without its own action view, but just an iconified view. The user can choose to pay with ether account, or pay it with ether + accorn account (the bottom text reads "Pay Accorn"):
And that iconified action, I think, can't be multi-columned.