What in a sentence, a plural or in the singular?

It is that what seems/seem to be causing it.

that = what seem/seems to be causing it, and refers to the noun clause

Marius Hancu wrote:

It, that, what all suggest singular

I've found 'what seem ...' in the following website.

Search Results

Oxford University Google Search

 It seems that even in provincial towns, the train stations are enormous. ... Surrounding the shower head, on a big chrome casing, are what seem to be large ...
www.robots.ox.ac.uk/~davidc/China/index6.html - 31k - Cached - Similar pages  

Surrounding the shower head, on a big chrome casing, are what seem to be large lights, giving it the effect of a futuristic surgeon's table. There are four switches on the wall, each labelled in Chinese.