begging for money and oil from what used to be their strongest supporter, outside of Iran, shows that the regime is on its last legs. The reasons why Russia has chosen to gradually relinquish its support for the standing regime is varied. Traditionally, or for at least the last decade, Iran and Russia have largely been allies of a sort. Using each other to counter the powers of the United States and Western Europe. However, Russia's priorities have changed since the very controversial re-election of Putin.
Putin, and the standing regime, have to contend with unrest that it did not have worry about for over a decade. It stands to reason that a regime already concerned about potential unrest doesn't want bring further attention via foreigning sources to itself. Perhapes the biggest issue is that Russia realizes that Assad is dead in the water. The Arabs want him out because his regime is part of the shadow network of power, that includes hezbollah and hamas, that Iran has within the region. Given that the Arabs want him (Assad) gone, and the United States as well, it stands to reason that the Russians realize that the cost of keeping the regime in power out weighs any benefits.
Moreover, the Russians do compete with the Iranians in the region. They aren't natural allies, and in fact, prior to the Shah coming to power and the collapse of the Soviet Union, they were adversaries. The Russian troops being sent to Tartus are being sent their to ensure that their facilities do not get damaged, or at least damaged beyound reason. Even now I suspect that the Russians are probably opening chanels to the rebels; offering them under the counter money for the promise that they will get to keep their important port in Syria.
It's an interesting peice of news, and one that should remind us that the US is not the only nation involved these kind of affairs. The amount another nation gets involved is only limited by its ability to do so.