Recurrence is common in pleomorphic adenoma because of:

Pleomorphic Adenoma: Local enucleation should be avoided  because the entire tumor may not be removed or the capsule may be violated, resulting in seeding of  the tumor bed. With adequate surgery the prognosis is excellent, with a cure rate of more than 95%. The risk of recurrence appears to be lower for tumors of the minor glands. Conservative enucleation of parotid tumors often results in recurrence, with management of these cases made difficult as a result of multifocal seeding of the primary tumor bed. In such cases, multiple recurrences are not unusual and may necessitate adjuvant radiation therapy. Tumors with a predominantly myxoid appearance are more susceptible to recur than those with other microscopic patterns. Key Concept: One of the characteristic is the microscopic presentation of tumour outside the Incomplete Capsule. So, the recurrence of pleomorphic adenoma is due to Incomplete Capsule. Reference: ORAL AND MAXILLOFACIAL PATHOLOGY, Neville, FOURTH EDITION page no 446,447