The current top rated chess960 player at Chess.com goes by the nickname posporov051560 and is rated 2253. His profile says that his 'Best Win' was against DavidKhachatryan, rated 2171. I found the game here: posporov051560 vs DavidKhachatryan (SP949 RKBBRNNQ). After the moves 1.Nf3 e5 2.d3 d6 3.e4 the players reached the position in the diagram.
In this position I would probably have been thinking about where to castle. It's not clear which side is more appropriate. To castle to either side, the two Bishops must move, and to castle O-O, the two Knights must also move.
Instead of moving any of the minor pieces, Black had something completely different in mind. He played 3...a5. White countered with 4.a4, a move that stops Black's further expansion on the Queenside. At first I thought this move was forced, but then wasn't so sure. Perhaps White can castle O-O, allow Black to advance Pawns on the Queenside, and use them as objects of attack.
After fixing the a-Pawns on a4/a5, both players followed up with the Rook lift (Ra3 & ...Ra6), then blocked further movement of that Rook with the b-Pawn (b3 & ...b6). It would be interesting to have the idea explained by one of the players, because I don't understand it at all. Whatever the reason, here's the complete game score, courtesy Chess.com. Note that neither player castled.
[Event "Let's Play!"]
[TimeControl "1 in 3 days"]
[Termination "posporov051560 won by checkmate"]
[Variant "Chess 960"]
[FEN "rkbbrnnq/pppppppp/8/8/8/8/PPPPPPPP/RKBBRNNQ w KQkq - 0 1"]
1.Nf3 e5 2.d3 d6 3.e4 a5 4.a4 Ne6 5.Ne3 Ra6 6.Be2 Nf6 7.Bd2 h6 8.Ra3 Bd7 9.Bc3 Nc5 10.b3 Be7 11.g3 Qf8 12.Ra1 g6 13.Nd2 Ne6 14.Ndc4 b6 15.Qf1 h5 16.f4 exf4 17.gxf4 Qh6 18.e5 dxe5 19.fxe5 Nh7 20.d4 Ra7 21.d5 Nc5 22.Bd4 Qf8 23.Qg2 Bc8 24.d6 cxd6 25.Qc6 Bd7 26.Qxb6+ Rb7 27.Qxa5 Bd8 28.Qc3 Ne4 29.Qb2 dxe5 30.Nxe5 Ba5 31.Rd1 f6 32.N5c4 Bc7 33.a5 Bc6 34.a6 Rb5 35.a7+ Ka8 36.Ra6 Bb7 37.Na3 Rb4 38.Nb5 Bxh2 39.Rb6 Rd8 40.Nc4 Qe8 41.Na5 Neg5 42.Rxb7 Qxe2 43.Qc3 Qxd1+ 44.Ka2 Rxb5 45.Rb8+ Rdxb8 46.Qc6+ R8b7 47.Qc8+ Rb8 48.axb8=Q+ Bxb8 49.Qc6+ Rb7 50.Qxb7# 1-0
The game appears to be one half of a mini-match. I found another game by the same players with the same start position, colors reversed: DavidKhachatryan vs posporov051560. The same a4 & ...a5 idea was used in this game.