Deep down those Spurs players are starting to realise that the last 2 seasons were probably as good as it is going to get. You finished second in a season where Leicester won the league, and then last year when City, United, Arsenal and Liverpool were all pretty poor. They are looking enviously at what players at other 'top' clubs are earning, and realising that at the very least City and United are likely to be a lot stronger than last season.
I think when things are developing, improving and the 'journey' is in full flight you can keep young, hungry, top class professionals happy - but if there is the slightest sense that the 'journey' has slowed down and they haven't quite reached the intended destination then they will start to get restless. This is a massive season for Spurs imho, if you take a perceived step backwards (which is quite possible in what is likely to be a hugely competitive premier league) then I see trouble ahead...