  We analyze the Belle data [K. F. Chen {\it et al.} (Belle Collaboration),
Phys.\ Rev.\ Lett.\ {\bf 100}, 112001 (2008); I. Adachi {\it et al.} (Belle
Collaboration), arXiv:0808.2445] on the processes $e^+ e^- \to \Upsilon(1S)\;
\pi^+\pi^-, \Upsilon(2S)\; \pi^+\pi^-$ near the peak of the $\Upsilon(5S)$
resonance, which are found to be anomalously large in rates compared to similar
dipion transitions between the lower $\Upsilon$ resonances. Assuming these
final states arise from the production and decays of the $J^{PC}=1^{--}$ state
$Y_b(10890)$, which we interpret as a bound (diquark-antidiquark) tetraquark
state $[bq][\bar{b}\bar{q}]$, a dynamical model for the decays $Y_b \to
\Upsilon(1S)\; \pi^+\pi^-, \Upsilon(2S)\; \pi^+\pi^-$ is presented. Depending
on the phase space, these decays receive significant contributions from the
scalar $0^{++}$ states, $f_0(600)$ and $f_0(980)$, and from the $2^{++}$
$q\bar{q}$-meson $f_2(1270)$. Our model provides excellent fits for the decay
distributions, supporting $Y_b$ as a tetraquark state.

High Energy Physics - Phenomenology;High Energy Physics - Experiment