The adjective doctored derives from the figurative use of the verb doctor, meaning per the OED:
To treat so as to alter the appearance, flavour, or character of; to disguise, falsify, tamper with, adulterate, sophisticate, ‘cook’.
The first citation is from the 18th century.
In case it helps you see how these things developed, here from the OED, minus the citations, are all the senses given for doctor verb, including this one:
1. trans. To confer the degree or title of Doctor upon; to make a Doctor.
2a. To treat, as a doctor or physician; to administer medicine or medical treatment to.
2b. transf. To repair, patch up, set to rights.
2c. To castrate (an animal).
3. fig. To treat so as to alter the appearance, flavour, or character of; to disguise, falsify, tamper with, adulterate, sophisticate, ‘cook’.
4. intr. a. To practise as a physician. (Usually in vbl. sb. or pr. pple.)
4b. To take medicine, undergo medical treatment.
Hence ˈdoctored ppl. a., ˈdoctoring vbl. sb.; also ˈdoctorer, one who doctors.
It should be pretty clear how the straightforward sense 2a turned into the transferred sense 2b, and thence to sense 3 by figurative extension.
This shows why it is important that senses be listed in the historical order that they came into the language, not merely by order of each sense’s currency or popularity.