Any idea ?
Yes. They used to lag behind because it was extremely dificult to make deposits there.
What has changed?
Well, now there is a LOT of money coming from other coins (eth, dash, ltc, ...) so trader there are faster reducing the spread.
If it is coming from other coins then the price of other coins should have been dumped but that isn't the case here
It's more complex than that. If we would experience a BIG pump on BTC you would see it more clearly. ALTS are continually pumping and dumping and there is always traders with liquidity on their BTC-e accounts. There's no need to be an inmediate connection between a rise in Bitcoin and a dump in altcoins.
When I used to trade BTC, then BTC-E / Stamp price correlation was an important indicator to me. If stamp was a lot higher then BTC-E, then it meant that new fiat was flowing in the market. When the difference started to decrease, then it meant that the flow of new fiat is starting to stop. People rarely use BTC-E to deposit or withdraw fiat. That's why it's always lower when new cash is flowing in and always higher when people are cashing out.
That was correct when BTC was responsible of the majority of the volume and liquidity, but now "deposits" can come from other alts, withdraws can go to them too. And funds on trader's accounts can go to Bitcoin or other altcoins. ie: a trader may chose to buy bitcoin instead of ethereum if he sees the spread makes it "cheap".
As I said, its more complex than that, but the dynamics of the liquidity flow has changed greatly in the past months.