Why do Grignard reagents add to propargyl alcohols?
The addition of Grignards to propargyl alcohols occurs via the alkoxide salt, after deprotonation:[1]



Here:


the reactivity may be explained by the stabilising influence of complexation;
the regioselectivity may be explained by the preference of a five-membered chelate ring over a four-membered one; and
the stereoselectivity arises because syn addition requires a five-membered ring with a trans double bond and is therefore precluded.


In fact, propargylic alcohols also get reduced by $\ce{LiAlH4}$ (Hans Reich's website has a page on it), with the same trans stereochemistry observed.



Reference

(1) Forgione, P.; Fallis, A. G. Magnesium mediated carbometallation of propargyl alcohols: direct routes to dihydroxydienes and enediyne alcohols. Tetrahedron Lett. 2000, 41 (1), 11–15. DOI: 10.1016/S0040-4039(99)01994-2.