Below is the definition of the OID used the retrieved the Transmit Discards in the IF-MIB:
The number of outbound packets which were chosen to be discarded even though no errors had been detected to prevent their being transmitted. One possible reason for discarding such a packet could to free up buffer space.
How the above OID is increased is at the discretion of each manufacturer. Because the IF-MIB might be limited in term of definitions for some manufacturers, they sometimes create additional MIBs to monitor the interfaces on their products.
If the following MIB is supported on your APs, it might give you more detailed information:
IEEE802dot11-MIB:ieee802dot11 - 1.2.840.10036
I would first think the APs is getting frames collision on the Frequency Band it is configured to talk...but you need to confirm this by checking the device itself.