Some of the causes of tumors in a dog include old age, poor oral care and breed of the dog, explains WebMD. Dogs that have not been spayed also have a higher instance of mammary gland tumors.
One reason for the development of tumors in dogs is that people are taking better care of them, according to WebMD. This leads to dogs having a longer life and reaching the age where tumors are more likely. There is also a genetic component; some breeds are more likely to develop cancerous tumors than others.
Some environmental factors can also cause cancer and tumors in dogs, states veterinarian Sophia Yin for The Bark. Cigarette smoke exposure can cause increased risk of nasal and lung cancers. Female dogs exposed to insecticides in flea sprays and dips are more likely to develop bladder cancer. However, the use of spot-on flea and tick products like Frontline shows no increase in cancer. Dogs living in houses with higher than normal magnetic fields are at higher risk of developing lymphosarcoma. Both cables above and below ground raise the risk.
There are conflicting reports about having pets around herbicides and pesticides. Since there is currently not enough information to tell if these products are cancer-causing, Dr. Joanne Intile for petMD recommends minimizing pets' exposure to them.