It's a big turnover, but I think the circumstances this season have been unusual.
Newcastle - Bruce was 100% guaranteed to be moved on once the takeover was completed. The fans seemed to hate him. However, he'd probably still be there now if Ashley was still the owner.
Spurs - Nuno never looked a good fit for that job, a very strange appointment in the first place.
Norwich - rock bottom of the league once again and without a win before yesterday. Farke had been there for a four and half years, so the board can't be accused of a knee-jerk reaction.
Villa - Smith had been there for three years and had done very well, but they definitely look to be on the slide this season.
Watford - a change of manager every season is standard practice there.