First thing to understand is the endemic nature of doping within cycling’s past and recent history. Reading Christophe Basson’s or Tyler Hamilton’s books will give a fair idea from both the clean and dirty sides of the sport.
Secondly, sky came onto the scene in 2010 stating an ambition to race and win clean. 2 years later they went further, stating any member of the team tainted by doping would be thrown off the team. Bye bye Michael Barry, Sean Yates ect… but they kept a hypocritical stance, keeping servais knaevan and leinders (one of cycling’s notorious doping doctors) in the team. This lack of transparency led to questions, easily swept away by Brailsford et al, but the drip effect had started.
Fast forward to 2012. Froome exploded out of nowhere, finishing second in the Vuelta (having been thrown out of the way giro a couple of years before for holding onto a motorbike). Wins the tour in 2013, rumours of a blood disease with possible unethical treatment appears and never really goes away, the tue falsification in the criterium dauphine in 2014, inhaler consumption whilst racing at another race adds fuel to the win at any cost fire.
The following two years see a dripping of accusations and implications of wrong doing with only minor criticism (Paul kimmidge being to the fore) then this whole Russian hacking thing blows the lid clean off.
Imo, wiggins doped to win 2012, and Froome has won each of his tour wins dirty. The gathering snowball effect of facts and stories have knocked down every excuse sky have used so far. Why wait until today to tell the fawning cycling press that the mystery package contained a legal substance?
Tl, dr but dig around and there’s so much information out there