Mites in the home can come from animal hosts (rodent and bird mites), outdoor environmental conditions (clover and concrete mites), and plant hosts (spider mites). Some, such as dust mites, are there all the time.
Before determining WHY you have mites, you first have to determine WHAT type of mites you have.
Because of the various sources, it is critical that mites are identified by a qualified specialist, including University and museum experts (NOTE: pest management companies, friends and social media such as Facebook, may be unreliable sources of identification, especially for mites). We recommend The Cornell Insect Diagnostic Lab and their instructions for collecting samples for identification: To collect tiny insects or mites, gently wipe up with a small piece of slightly-damp paper towel or tissue, and put in a ziplock bag or tight-lidded container and prepare for mailing. Digital photos are an option but rarely provide the detail needed.