not too uncommon. NWA did this often on the MSP run (320) if winds permitted it. TWA also did it with the MD-80 to STL.
I think it has something to do with the noise abatement climb gradient and high weight takeoffs.
This procedure is also used by all aircraft during Santa Ana wind (offshore) conditions.
I have done it flying DL from SLC (EMB-120) and to ATL (757) due to winds.