One will have to wait for some time to understand the specific reasons for the results that have astounded the UPA and the INC itself. My own take is that the recession has helped the UPA government. During times of recession and external threats like the Mumbai attack, people tend to stand by the ruling group. In times of rapid economic growth, high inflation and increasing disparities, the mood tends to be anti- establishment (or anti- incumbency). This is the first time since neo- liberal reforms were kicked off that a ruling party has been re- elected. The NREGA and the relative rural focus (even if it is the corruption/ subsidies formula as in Andhra Pradesh) also seem to have been factors in the revival of the INC.
The BSP’s consolidation over the years has also meant that the INC has become slightly more sensitive to Dalit voters. Even the symbolic display of solidarity like in the case of Rahul Gandhi going out and staying with dalit families has had some effect, as has his attempts at rejuvenating the INC in UP. The Left’s attempts to force feed industrialization in West Bengal have backfired.
The greatest threat for the next five years now are the INC and its economic policies. A weak INC and a strengthened UPA+Left+BSP would have been a better combination providing a counterweight of checks and balances. With the INC having hijacked the Jana Sangh’s economic policies since 1991, and with major losses for the Left, no major opposition to them, it could mean a return to right wing jingoism on the part of the INC/UPA.