Jeremy's got the final word on this one, but "up to 30 feet in a straight line" means exactly that, you can't zig-zag it back and forth. You'll either move it on them and stop, or run it over them. They make one save and either get dragged along or pushed out. #WotCStaff #dnd
— Dan Dillon (@Dan_Dillon_1) March 31, 2019
The spell doesn’t say the sphere stops if it rams a creature, so using it in a narrow hallway could force multiple saves. But yeah, if the creature can choose to be ejected sideways, the sphere will keep going past it. If the only unoccupied space is directly in the sphere's path, sure.
You can't "chase an enemy around" with it as a voluntary thing, though.
— Dan Dillon (@Dan_Dillon_1) March 31, 2019
Even then, as a DM if the sphere is moving I'd probably let them pick the space they were already in, which the sphere is vacating as a valid unoccupied space to land in.
— Dan Dillon (@Dan_Dillon_1) March 31, 2019