U-turns could be called for in the case of unexpected prohibitive weather at the destination, and the airline elects to return to origin rather than divert somewhere else.
This happens between Auckland and Christchurch. The development of fog unexpectedly at Christchurch will result in AKL-CHC flights doing a U-turn to return to Auckland.
There was a case of this on 19 July 2010, much celebrated by the local chemtrail community.
https://chemtrailsnorthnz.wordpress.com/2010/07/30/good-video-footage-of-chemtrails-over-nelson/
and see the link to stills here:
http://chemtrailsoverkaikoura.ning.com/photo/photo/slideshow?albumId=4744723:Album:3355
Here are the METAR from NZCH showing why these aircraft made U-turns.
METAR NZCH 181700Z 04005KT 9999 BKN010 05/04 Q1015 TEMPO BKN009 RMK RMK SUGARLOAF 11005KT
METAR NZCH 181800Z 33004KT 9999 BKN010 03/02 Q1015 BECMG SCT020 TEMPO BKN009 RMK SPECI CLD SUGARLOAF 27006KT
METAR NZCH 181900Z 29003KT 9999 FEW010 02/02 Q1015
TEMPO BKN009 RMK MIFG SPECI CEASES SUGARLOAF 25008KT
METAR NZCH 182000Z 23005KT 9999 FEW010 02/01 Q1016
TEMPO TL2100 BKN009 RMK SUGARLOAF 26010KT
METAR NZCH 182100Z 22003KT
0400 FG SCT010 02/02 Q1016 BECMG 9999 NSW TEMPO 0500 FG BKN002 RMK SPECI VIS SUGARLOAF 26013KT
METAR NZCH 182200Z 23003KT 200V260
0200 FG VV/// 03/03 Q1017 BECMG FM2300 9999 NSW RMK SPECI VIS SUGARLOAF 27016KT
NZCH 182300Z MISSING
METAR NZCH 190000Z 28002KT 0700
0500W FG VV/// 05/05 Q1016 BECMG 9999 NSW RMK SPECI VIS SUGARLOAF 29009KT
METAR NZCH 190100Z 36002KT 9999 BKN004 08/07 Q1016 BECMG SCT020 RMK RMK SUGARLOAF 30010KT
METAR NZCH 190200Z 05003KT 010V100 9999 FEW006 10/07 Q1015 NOSIG RMK RMK SUGARLOAF 30008KT
The TREND at 2000Z was for the stratus to clear after 2100Z (TEMPO TL2100 BKN009).
But at the following hour (2100Z); visibility 400 meteres in fog! Totally unexpected. Back to Auckland, folks.
It didn't clear til after 0000Z, 3 hours later.