I entered a Twitter pitch contest recently and got a few 'likes' from agents. I sent off the requested material, and received rejections from most of the agents a couple of weeks later. Then today, I got an email from a different agent at one of the agencies than the one I sent my material to. A junior agent.
She loves the book and wants to schedule a call to talk about it. What should I do? I'm excited to have come this far, but I'm not sure I want to sign with this junior agent, or even why she's the one who has read my book when I sent it to a different agent.
It seems like you have a couple of different questions in there, so I'll try to answer them one at a time.
Firstly, junior agents in larger agencies often read the slush coming in. Busy agents rely on interns and junior agents to read the slush and dig out the gems for them to read. It sounds like your Tweet caught an agent's eye, and she asked her trusted junior agent to read the material. And said junior agent loved it.
Maybe the other agent (let's call her senior agent) also read the manuscript and didn't love it as much, but saw that it has potential in the right, passionate hands. Junior agents don't have a ton of clients already and are building their list of authors and contacts. They have more time for individual authors and projects. If senior agent believes the book is good enough, and may sell, she might have suggested junior agent takes it on and will then offer support and advice as junior agent goes through the process of trying to sell it.
Junior agents may not have the same number of sales as a more senior agent, but they often have more time to spend on individual projects than their more senior counterparts. They are just beginning their careers and don't have a large number of authors competing for their time and attention. If a junior agent has the support of an established agency or a single experienced agent, signing with them might be the best career move you can make.