Commands sometimes execute in command blocks when loaded from a structure block. This seems to happen for those command block that are powered in the structure.

An example structure that shows this behaviour is attached as g16.zip. This is a machine that teleports the player around the map to generate chunks. When this structure is loaded, it teleports the player three times (and executes other commands from chained command blocks).

Unpack the g16.zip into a test world.
Load "g16" in a structure block -> player teleports.