One embarrassing attendance. Even this season, the majority have still been over 55K.
And I'm sure that you're fully aware that there is a specific reason for the dip in attendances - namely the continued use of Wembley. Spurs fans have had enough of it. It's an awful place to watch football for anything other than a one off game. Not to mention that Spurs fans are probably being asked to pay the highest average prices in world football, with tickets for similar seats (but far worse views) costing what they would have at the new stadium had it been in use. So, much as City fans have turned their back on the Champions League in their droves because of their dissatisfaction with UEFA, so Spurs fans have turned their backs on Wembley in their droves.
And much as City fans always return in their droves for Premier League games, so Spurs fans will return in their droves once the new stadium opens. Simple as that.