Russian president now demands the departure of more than 700 diplomats of the US from Russia so that the numbers in both countries are equal. It is very apparent a revenge of something. But why did Russia permit so many US diplomats stay in it till now? Did it want to get some business opportunities through these people? At least it could have gained some foreign curriency through their everyday life?
The original expulsion of Russian personnel was by the Obama administration, in late 2016, in retaliation for claims of interfering with our Presidential elections.
In the public statements, Trump had given every indication of having a more cordial view of Putin and the Russian administration, and part of the allegations being looked into are whether some of the communications with Russians that were concealed/lied about included promises to roll back or negate those actions by the Obama administration. In either case, it appears that Russia decided not to retaliate, tit for tat, so to speak, in order to not escalate the situation in the hopes that, by being relatively docile in their actions, they could maximize the chances of the policies being changed with a new administration and both houses of Congress being the same party as that new administration (and the opposite of Obama's party affiliation).
However, with the entire Russia affair refusing to quietly go away, members of Congress seem fairly reluctant to take a stand for leniency for Russia, and, indeed, the Congress just passed, in both houses, a bill formalizing additional sanctions against Russia. The Trump administration has decided not to further raise their profile or raise additional questions, and indicated he will not exercise a veto.
It appears that Russia has decided that their tactics of not retaliating in-kind has not gained them any kind of hoped-for benefit, so they are moving forward with a more expected reaction of in-kind retaliatory actions.