Personally, and this is just my opinion, I think there is some problem somewhere in the Magellan calculation of the rotation of Venus, probably to do with their ephemeris correction technique.
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The alternative is that Venus sped up prior to Magellan arriving at the planet which is why pre-Magellan measurements as well as Venera-Magellan combined measurements showed a rotation rate slower than the Magellan-alone value and identical to the current value measured by Venus Express.

In that case, it's not simply a matter of Venus slowing down, nor does it mean that something strange is happening now that didn't happen before. Rather, it means that Venus' rotation is not stable over long periods of time and it was exactly this slow prior to Magellan, a fact being overlooked by the news media.