The major product of the following reaction
When you form the allene, the methylene hydrogen has enhanced acidity because deprotonating it leads to a carbanion in which the negative charge is delocalized:

$\ce{CH3-CH=C=CH2} - \ce{H^+ -&gt; CH3-CH=C=CH^- &lt;-&gt; CH3-C^-H-C#CH}$

The proton then reattaches to the opposite end of the allene chain to form a more stable product.  In effect this is a (strong) base catalyzed tautomerization.