Raid Timer Countdown: egg hatch or start time?

If this is answered somewhere else, let me know, and I'll delete this, but I need to know this because I haven't pieced it together on my own yet.

I'm watching an egg countdown on a tower right now, and I know that the timer on that is until the egg hatches and the boss is revealed. As soon as that egg hatches, that's when the first group can join in, right?

I ask this because I drove by another raid the other day; saw it was a Lapras, and decided to turn around and join it. From there, I had to wait about 90 seconds to join in.

Was that me waiting to join in on the next battle? A cool down time, so to speak, between battles?

I know this sounds dense, but I'm still trying to piece this all together, and Reddit search isn't helping too much.