They were probably cycling already and it’s probably just luck more than anything that they’ve cycled together. The ram effect occurs when a ram runs with ewes that aren’t cycling and have been out of sight and sound of rams for a minimum of 6-8 weeks, runs with them for a fortnight, bringing them into season. If your ewes were with him till 1st Oct (been in all year??) and he served them the day or day after he returned, I’d say it’s just luck.