[Baz]

It's a misconception that low voltage will not produce heat when wires are crossed. That is what is happening there. You have to go back over everything and find the short.

Can be tedious but with your simple system not bad. Just be thorough and you'll find the issue.

Potential issues: transformer, cable, LED fixtures, LED bulbs, connections.

I don't heat shrink my connections - I use wire nuts filled with dielectric grease.