Thresh hooks on Galio during his E

comes in for a gank and I throw a hook at him as . The hook connects the second he starts the E animation ( the part where he takes a step back ). His animation cancels and I start ( dragging? ) him. When the dragging animation finishes, his E continued and I get knocked up. Is this a bug or is it supposed to uninterruptible? If so, then why did the drag animation caused by my hook start?