Think they really made a lot of new fans via the festival circuit in 2016ish. Remember their set at Primavera that year was incredible and seemingly everyone in attendance had the time of their lives (probably the 2nd most attended set after Radiohead that year too). Seem to have made the leap from 3-4 on the bill that time around to headliner status now (Coachella, Primavera, etc).
Brits win, Rihanna cover and the use of Let it Happen on just about every commercial under the sun probably didn’t hurt either.
Do often wonder how they go about identifying when indie bands like them, Arcade Fire, and The National have become mainstream enough to play the O2. Before presumably record sales would give a good indication but now all those stats have been warped by illegal downloads.