It looks like Van Wely is not having a good tournament at Biel. I think in the last year GM Van Wely has not performed very well, and his star has fallen during that time. At Biel, this is his third loss in four rounds.
Here is the position at move twenty-six for black. Van Wely seemed conscious of the threat on b7 and b6, a one-two combination punch by the white rook and queen. To address this, Van Wely played 26...Rd6 so as to prevent the subsequent check on b6 by the queen. Evidently, this was not enough. Onischuk applied more pressure via 27. Bd5 with the threat of 28. Rb7+ followed by the double attack 29. Rb8+ and then Ka7 30. Qb7#. Van Wely was quickly running out of rope although he was one piece ahead. Black played 27...Rd5 but it was not enough. Onischuk secured his win with 28. Rb7+ Ka8 and the very fine move 29. Qb6, threatening mate in two ways---30.Qa6# and 30. Ra7#. Game over. 1-0