Flyers
Causes of flyers:
1. Fault of the shooter from inconsistent rifle hold, flinch, etc. These shouldn't really be called flyers but poor shooting.
2. Fault of the scope from loose elements or loose mounting.
3. Fault of the gun from a poor crown, a barrel that warps slightly from heat, poor bedding, poor barrel, etc.
4. Poor ammo with inconsistent powder ignition, bullets that are out of balance or inconsistent in weight, etc.
Another fact to keep in mind, one shot has no statistical significance in the accuracy department. Two shots have very little significance and 3 shots aren't a whole lot better. To know whether you have flyers or just big groups you need to either shoot more shots per group or shoot more 3 shot groups.
Not too long ago I shot a 3 shot group with an M1 Garand with open sites at 100 yards that went into about 0.75 inches. There were no "flyers" but that single group doesn't mean anything about the consistency realistically obtainable with the rifle. I also shot a 3 shot group at 100 yards with a 458 Lott that went into less than an inch; once again not a fair trial on accuracy.