Attila died on his wedding night, possibly murdered by his bride.
What uncertainty does the word possibly imply there about the death of Attila?
The other source is a recent book by a philologist who argues from literary analysis that Attila was actually murdered. See http://worldhistoryconnected.press.illinois.edu/3.3/br_may.html for a sympathetic review of the book. However, the review also points out some of the weaknesses in Babcock's theory.
Since no new evidence is likely to come up, the jury will probably have to remain out on this on. So it is quite understandable that the Britannica chose to apply Occam's razor to this question.