And I have my answer:
From Journeycheck
16:10 Weymouth to Westbury due 17:39 will be cancelled.
This is due to a shortage of train crew.
Additional Information
Passengers for stations to Westbury, please wait for the 17:53 departure time for the next GWR▸ service; this has limited rail replacement road transport from Weymouth for Upwey, Dorchester West, Yeovil Pen Mill, Castle Cary, Bruton, Frome and Westbury.
Well it’s such a lovely afternoon, why would you want to leave early?
17:53 Weymouth to Bristol Parkway due 20:55 will be cancelled.
This is due to a shortage of train crew.
Additional Information
Limited rail replacement road transport from Weymouth for Upwey, Dorchester West, Yeovil Pen Mill, Castle Cary, Bruton, Frome and Westbury has been arranged.
Please check the stations Onward Travel Information poster and wait for the replacement road transport at the designated stop.
The replacement road transport may run later than the advertised train times owing to the additional time taken by road between stations and the time required for loading and unloading at each stop.