This seems to mirror what I see for most ladder bugs.the problem with statements like this are that there are 2 distinct different bugs with ladders.
the first bug was getting stuck on ladders. this was caused by a misalignment in the ladders after they moved up to DX9. keeper had to go back through every ladder and realign them. these bugs have been pretty much eradicated from the game.
the second bug is a little trickier and affects every ladder in the game. this is the bug that causes you to rubber band back to the bottom of the ladder every time you start climbing it. the only way to fix it is to step off the ladder and start over.
theory 1 - your client recognizes that you are on a ladder but the server does not. when the new climb coordinates are sent to the server, it rejects them as "invalid" because you can't walk up a vertical surface so it starts you back on the ground. this is repeated until you step off the ladder and get back onto it.
reasoning - this bug only occurs when you try to get on a ladder by running into it (your feet are on the ground). if you jump onto the ladder you never see this bug.
theory 2 - you are caught in an out of synch loop with the server. basically, the client is sending an update for rung 3 when the server thinks you are on 1. the server sends back a reset message. rinse repeat.
reasoning - very dependant on lagged messaging.