I presumed the resemblance to the Racnos was deliberate (spiders v scorpions) and was waiting for the obligatory one liner somewhere that they came from the planet next door or something (like the Ood and the Sensorites).

As for why they were so clumsy, they had to be otherwise there was absolutely no chance of outrunning them!

It seems that every season is contractually obligated to include one absolute stinker of an episode, and this was it.