Fighting back against supermarkets

Have you noticed how brand named teabags are taking it in turns to be priced at approx. £3 (for 240 packs) in supermarkets?

Why are two or more brands not matching the £3 price at the same time?

It's as if they are colluding, surely they wouldn't be doing that? Would they?

If I owned a major teabag brand and one of my rival brands was selling their teabags for £3 for a 240 pack, I'd instinctively match their price to maintain sales and be competitive.
Factor in that the big supermarkets (over 70% market share) have deals on the same brand of teabags at the same time and all rotate their offers between each big brand, I don't see much "competition driving down prices for the consumer," do you?