Obstruents

Many thanks to all those who responded (listed below). 

The main reason put forward for a possible explanation of this stress pattern 
was analogy with other “ob-“ words such as obstruct, obtain, observe, etc, all 
of which have second syllable stress.

John Wells, however, pointed to another pattern which one would suppose 
might have blocked the one just referred to; that is, the influence of other “-
uent” words, such as affluent, congruent, constituent (and similar such as 
radiant, variant, etc). (By the way,  thanks to John for putting this on his 
blog.)

My main reason for wondering whether I’d missed a US pronunciation, 
however, is the fact that the speaker is a speech pathologist of over 20 years 
experience (and thus trained, we’d hope, to listen really carefully to speech) 
and a current doctoral student (thus, we’d hope, having encountered the 
word in question reasonably often), coupled with the fact that I’d been using 
the word in doctoral seminars over several weeks....

Maybe this realization was influenced by the stress pattern in 'disfluent'?

A couple of other points: thanks to Bob Ladd for reminding me that, of course, 
DEbris (as opposed to d@BRIS) is actually a British English stress shift back 
from the French original.

And thanks to Natasha Warner for allowing me to share some annoying 
student pronunciations/versions of phonetic terms. These include forMANT, 
FORmat (both for FORm@nt), constonant, approximates, and dipp-thongs......

Thanks go to:
John Wells, John Maidment, Natasha Warner, Bob Ladd, Mark Tatham, David 
Ward, Mark Jones, Ian Crookston, and Allard Jongman.