What is a DSL Link Retrain?

Our office has three ADSL lines running to it.

One is provided by BTr through a Home Hub.
The other two are provided by Twang (as was) and are load balanced.

We've noticed for some time that the load balanced pair drop out a couple of times a day but the other line is rarely used and we thought it was fine. We've just found out that in fact when the two main lines glitch the Home Hub reports a 'DSL Link Retrain'. It doesn't report LOF or LOS though. Those are both very low (3 LOS in 23 days).

I suppose it's likely that all three lines run in the same bundle since we're at one end of a village but I'm surprised that a single event (in a village) could actually be that big.

The attenuation on all three lines is 54db and the Home Hub stats show a 4.4Mb sync and speed tests show around 3.6Mb/s which seems very fair. Unfortunately the main lines go into a Cisco box and I have no idea how to get the stats for that.

Thoughts?

a link retrain could be triggered by a low SNR event, may not need to have lost anything.