Stassi hit a mile high fly ball to deep right. The A's right fielder was waiting for it at the fence, but a fan in the front row made a basket catch. His hands may have been a foot or two inside the top of the fence (hard to tell from the video). The fielder immediately called for interference and the umpire gave it to him. HR waved off, out.
If somebody in the front row, reacting naturally to a ball that appears to be coming right at him, can interfere without leaning in or even fully extending his arms, those seats are too close to the field.
EDIT: The second angle in the video below shows his hands a foot or more over the top of the fence. But he could literally touch the top of the fence from his seat, which seems silly.