This was in issue, to a lesser extent, in pre-Engineers ED but it is now quite common. Pre-Engineers, I got tired of being the fox so I got rid of my freighter T7 and bought a Python, it's got cargo capacity, decent speed, range and can fight when cornered. I eventually a-classed it and got to the point where I looked forward to interdictions.
Engineers dropped and an a-class ship was tissue paper to the NPC's initially. The NPC's were deprived of modded weapons and I slowly hardened the Python with mods. The drawback is cost, 150+ mill CR plus the time invested for mods. But, IMHO, it is the best way to survive.
The instances described in this thread are common, if you're carrying engineer bits and are running a mission that has hostiles attached, the odds that you will be attacked are enhanced; if you have a bounty, another layer, Anarchy system, another one and so on.
Point is the odds are stacked against traders unless they are in fully-combat ready armed merchants.
There are common instances of instant interdictions just as you come out of hyper, with no warning. To better your chances, close throttle and submit to avoid the stupid spin cycle in the useless mini-game. This allows for faster FSD spin up and betters the survival/escape odds. boost away and high jump ASAP to a nearby system, this usually breaks the cycle.
Run. rabbit, run (until you get that Python).