Competitive 'Wittig' vs 'Peterson' C=C bond formation
Seems like the silicon atom is more oxophilic as compared to the phosphorus atom. This could in principal be explained by the higher strength of the $\ce{Si-O}$ bond, as indicated by the dissociation energies $D$ (see http://www.wiredchemist.com/chemistry/data/bond_energies_lengths.html):
$$\begin{array}{ccc}
\hline
\textbf{Bond} &amp; \textbf{Dissociation energy / kJ mol}\mathbf{^{-1}} &amp; \textbf{Bond length / pm} \\
\hline
\ce{Si-O} &amp; 452 &amp; 163 \\
\ce{P-O} &amp; 335 &amp; 163 \\ \hline
\end{array}$$
The $\ce{Si-O}$ bond is stronger thus given an explanation why the Peterson olefination is favoured.
It seems that the bond lengths $r_b$ are the same, though.

By the way, a nice discussion of oxophilicity was given recently in Inorg. Chem. 2016, 55 (18), 9461.