One of the main reasons is that the oligos are not completely dissolved. If you are not able to completely dissolve oligo, then you may add NaOH to raise the pH to 7.0. Another recommendation would be to dry the oligo completely and re-suspend in TE at pH7.0.
10mM Tris-HCL, 1mM EDTA can also be used as a re-suspension buffer but EDTA may inhibit some experiments.