In case of an asynchronous hand-over related channel activation, 3GPP TS 48.058 Section 4.1.3 is very specific:

BTS starts transmission immediately on the main channel in the indicated mode and with encryption if so indicated. If
the MS Power element is present the BTS may start transmission also on the SACCH.

When receiving a correct access burst with the correct handover reference, BTS starts the normal reception process on
the main channel in the indicated mode and starts receiving (and sending if not started earlier) on SACCH. Deciphering
is started if so indicated. The handover detection procedure towards BSC is also started.

However, as uncovered by an upcoming BTS_Tests.TC_sacch_chan_act_ho_async test case, we appear to be activating the SACCH unconditionally from the first moment.

The problem here is quite obvious: Until we have received the access burst from the MS, we don't yet know the timing offset, and hence the timing advance that we should advertise in the downlink SACCH. If we start SACCH transmission too early, it means that a wrong TA is advertised, which may be picked up by the MS, which will then apply a wrong TA value -> boom.