What could cause root rot after repotting?

[tsun1108]

I had a couple of phalaenopsis which I bought from garden centres and they came in bark with tag saying they were last potted late 2011.

After flowering they were doing fine. When comes summer - am in NZ so it's summer now for us (and they are showing sign of new leaf & root), I decided to repot them in new bark.

Roots were good when I repotted, and I cut some of the old ones with sterile scissors (I worked in health related field so I have access to proper sterile tools).

Repotted them in new bark which had been pre-soaked the night before.

Didn't water for at least 10 days, thought kept them a bit on the drier side.

So this is the method I repotted my other plants in the past, they all did well with new roots and all re-spiked.

And 2 weeks later, both of the plants leaf started withering and I am talking about all of it..... so I pulled them out and all the healthy roots were now gone! Black, shrunken with even fungus on it.

What could've caused it? The only thing I can think of maybe the bark I used was not fresh enough seeing I bought them last year but they have been kept outside.