Does anyone know what has caused this?
All the other seedheads were wind blown in the usual way or still in tact, like the one in the background. This one seems to have been shaped like this on purpose. It is in the middle of a pond so presumably wasn't man made. I am assuming some creature has used the seed head as insulation by wrapping it round itself or its eggs or something and would be interested to know, if anyone knows for sure.
I'm not an expert but suspect that this is just the pattern of the seed head opening on this particular plant. Were you able to check on the same one a day or so later? Or did it rain before you saw it that might make it too damp to open properly?
There certainly are larvae which feed inside Typha seed heads, such as the moth Limnaecia phragmitella
- http://en.wikipedia.org/wiki/Limnaecia_phragmitella. Also, search for that name here - http://www.carmarthenshiremoths.co.uk/2014_03_01_archive.html
But the symptoms are much more tatty than in your photo.
(Note: bulrush wainscot, Nonagria typhae, feeds inside the stems rather than flower/seed heads; ...as does the reed stem borer (sawfly), Calameuta filiformis)
I suspect that the seed head in your photo was damaged early in its development, leading to abnormal development. Damaged by what - I don't know. But I can imagine it could be a complicated scenario, eg. a stem-boring larva (not necessarily doing sufficient damage) being searched-for by a bird (which might have done the damage in gaining access to the larva).