How did that happen?

I Have No Idea, I Went To Fix The Aurora, I Then Went To My Base And When I Went To Locate The Blood Kelp Zone To Find Materials For A Prawn Suit I Found The Reaper In The Shallows While I Was Halfway There

He probably saw you and stalked you.

Yep...

It was at this moment he knew, he fucked up

That happened to me...

I was in a Prawn and it just followed me after I got cyclops fragments.

Followed me back to my base...

Ate the Prawn when I was going to get some creepvine seeds... (it was living in the kelp forest nearby)