There are three reasons for unexpected chorus/flanging:
1/ You have am audio effect in the chain. Maybe on the instrument on the channel strip but may be on the output, bus or group strips.
2/ You have a midi effect in the chain. This could be the modulator plug in or it could be in the environment, for instance this could be GM Mixer or if you use an external midi instrument has chorus controls
3/ And finally classic phasing/flanging occurs when you double up a track - if you accidentally have two tracks with the same sound OR if you output a parallel path which are slightly out of phase
Have a look - I'm sure there are other reasons but I'd bet 90% of the time its one of the above.
Good Luck