Seasonal sow pheromones can indeed trigger another sow close enough to her own. In larger groups you often find (like in larger groups of women living closely) that they will have their season in two clumps; one larger one with most of the sows and then another smaller one with the sows whose own seasons and pheromones cannot interact with the larger bunch.
Sows tend to have the strongest seasons around 6 months of age at the height of teenage (parallel to the life time testosterine high of boars) and then can have the odd strong season, depending on fluctuations in the own hormones although the strength of seasons can vary enormously from barely or not noticeable to very noticeable. Unless you have a sow with naturally higher hormone levels, under-sows in a group tend to have a less strong season than top sow.
I have documented this phenomenon in videos in this guide link here:
When Sows Experience A Strong Season (videos)
I have experienced 3 days long mega-seasons in my piggy room as the strong pheromone output would trigger all possible sows to also have a strong one that are within reach of being triggered in their own oestrus cycle. It is totally mad, smelly (although as humans we cannot fully appreciate cavy pheromones) and everypig is rather drained after that.
If one of your sows seems to be nonstop in season, please have her checked for ovarian cysts. The most common age is 2-4 years but they can happen quite a bit earlier or later. The older a sow gets, the more common non-hormonal fluid filled cysts become who cause problems with the size they can grow to but they do not cause any hormonal issues.
Here is more information on ovarian cysts:
Sows: Behaviour and female health problems (including ovarian cysts)
