  We point out that the existence of Glauber gluons in the $k_T$ factorization
theorem can account for the violation of the Lam-Tung relation, namely, the
anomalous lepton angular distribution, observed in pion-induced Drell-Yan
processes. Radiations, that balance the lepton-pair transverse momentum, cause
the responsible spin-transverse-momentum correlation in the Glauber-gluon
background. It is argued that the Glauber effect is significant in the pion due
to its unique role of being a Nambu-Goldstone boson and a $q\bar q$ bound state
simultaneously. This mechanism is compared to other resolutions in the
literature by means of vacuum effects and Boer-Mulders functions. We propose to
discriminate the above resolutions by measuring the $p\bar p$ Drell-Yan process
at GSI and J-PARC.
