When they are competing for a deal, they aren't talking to each other in this way. But it does happen that they will refer or intro you to others even if they themselves decide to pass, as long as they pass for a technical issue and not a bad idea/market/traction etc.

In your case it could be coincidence but I doubt it. More likely the junior VC (or the partner he spoke with) reached out to a friend or someone they know at the other firm(s) and said hey this looks interesting but we haver to pass for X reason. A VC's goal is to make money, even if you are a competitor to one of their holdings you being in market with a great product helps their holding too. People think the lack of competition is great, it isn't, having competition in market is much better. It lowers cost of client acquisition for everyone and increases awareness so it isn't all on one company (or VC) to fund. VC's know this.

My anecdotal experience is the junior VCs tend to form small groups and share deals with eachother. This works because they are measured on deal flow so it is more important than competing. They know who the senior partners are competing the hardest against so they usually won’t collaborate with those firms as much.