[Chuffy]

I may cry....

Planted out two GAs from the garden centre last year. One in particular turned into a leafy monster. I didn't cut them down over winter, I just mulched round the base with straw. The big one had a nice little globe coming and I was looking forward to a decent crop this year. Then I went out on my daily tour of inspection and saw that it had fallen over. When I looked closer it turned out that the base had rotted right through apart from a 1"strip of stem.

Needless to say I'm very disappointed. What did I do wrong?

[Stripey_cat]

The frost will often cut them down to the base anyway - they'll regrow like any other herbaceous perennial.  If you mulched right up to the stem, it probably trapped water against it and rotted it (or maybe the slugs hid under the mulch and chewed the stem, letting in the rot).  Don't fret - mine are only a couple of inches tall and I still expect them to crop this summer. 

[woodburner]

What did I do wrong?

Nothing really. Nature did what you didn't.
I don't know about GA particularly but for perennials in general, rotting stems can sometimes introduce rot to the plant. But IMHO most of the time the advice is just left over from the tidy minded victorians. Personally I cut off flower stems, but leave any healthy leaves.