This is crazing of the finished render coat, assumeing that materials were batched correctly I would think that perhaps the rendering was done in warm weather which has caused the mortar to cure too quickly. Other causes could be lack of sufficient hydrated lime in the mix, finished coat of stronger cement to sand ratio than in the scratch coat or applying finish coat to a unsufficiently wet down scratch coat again causing premature cureing of top coat plasters. Can cause problems long term if not painted.