The US decision to impose a second round of sanctions against Russia over the Skripal poisoning was the result of pressure by powerful forces in the US which are against the normalisation of relations with Russia for political domestic reasons, and because US elections are approaching, while Trump seeks instead to improve relations with Russia.
Young people in Moscow had no serious reason to protest in prosperous Moscow. In fact, in the context of young people participating in the Yellow Vests protests in France, the protests in Hong Kong, and the protests against Brexit in the UK, it became fashionable in Moscow. The protesters have no other motivation than enjoying concerts and partying during these protests.
When people really want to protest, they do it like the football fans in 2002 – with violence. Or like in the 1991. And, for example, the Soviet people protested in silence.
Recurring pro-Kremlin disinformation narrative about the protests in Moscow.
The protests in Moscow started in July after the election committee banned opposition candidates from running for the elections to the city council - the Moscow City Duma, disqualifying their ballots because of what officials claimed were irregularities in the 5,000 signatures each had to gather to run. The protesters also demanded the right to fair elections and the right to free assembly.
See here for EUvsDisinfo analysis of disinformation surrounding protests in Moscow.