I feel the reason to switch off the FD bars is basically when the pilot is not following the FD commands. This can happen at time of TCAS or any other event.
Both FD are switched off to avoid confusion.
and
From Airbus FCTM :
- Both FDs must be disconnected once APs are disconnected:
‐ To ensure autothrust speed mode
‐ To avoid possible confusion between FD bar orders and, TCAS aural and VSI orders.
This is not the main reason you turn them off during a TCAS RA. I hope most pilots would not be confused if an RA was saying climb but the FD bars showed a descent due to open des mode.
The reason is, if you are in open descent and disconnect the AP and pull back for a TCAS RA no thrust will increase, your speed will bleed off and you wont climb very far. Turning off both FDs puts the autothrust into speed mode so as you pull back the thrust will increase.