For me it all started when rumours came out that the board/saints where taking the club in a different direction, if I rememer correctly Guan eluded to this at the time, subsequently we then hired Puel.
My interpretation (I could be totally wrong I know) is that after Koeman they decided to hire a lower profile manager with less demands, a manager who was more compliant with the southampton way rather than building up his own profile via success.