  We study the two step sequential one pion production mechanism, $np(I=0)\to
\pi^-pp$, followed by the fusion reaction $pp\to \pi^+d$, in order to describe
the $np\to \pi^+\pi^-d$ reaction with $\pi^+\pi^-$ in $I=0$, where a narrow
peak, so far identified with a "$d(2380)$" dibaryon, has been observed. We find
that the second step $pp\to \pi^+d$ is driven by a triangle singularity that
determines the position of the peak of the reaction and the large strength of
the cross section. The combined cross section of these two mechanisms produce a
narrow peak with the position, width and strength compatible with the
experimental observation within the approximations done. This novel
interpretation of the peak without invoking a dibaryon explains why the peak is
not observed in other reactions where it has been searched for.
