Because you let it flower - if you see a flower stalk appearing, it should be removed before it opens, and certainly before it sets seed.

The aim of any and all plants is to set seed. As Bamboo says at the first sign of a flower stalk on rhubarb remove it by cutting off.

Good point, Moongrower - I neglected to point out that all plants will try to reproduce themselves ,one way or another.

dont they all {nearly put dont we all } :o)
animals too.