The AI generally can send missions faster (without the Research, as I saw in Diplomacy), probably to help up your relationship with either of them (also help your economy if you manage to finish their quests) and give it a boost. I'd just say it's something intentional, to help boost your relationships with other AIs faster, though this works between them as well, turning the higher difficulty level AIs into near-invincible allies as well 
Could also be a buggy, just as in Diplomacy, sometimes, AI would send you quests in Locked Teams 