How have I killed my bedding plants with strulch

New gardener so please be kind

I bought some Pansys and primroses to plant out, I planted them in a mix of pots and the borders. I ordered them online and followed when to plant out and where I could put them. I left it as that for a week or two due to the weather.

I then decided to go around and spread strulch around as I’d read that was a good thing to do…

HOWEVER, I’ve clearly done something wrong as since the strulch went down everything that was once happy is now dying

Where did I go wrong here?

I’m gutted as I was really excited about the Pansys