Although there are rather simple rules determining the pronunciation of "the", native speakers quite often deviate from these rules (including, e.g., TV shows). According to the Longman Pronunciation Dictionary,
The EFL learner is advised to use [ðə] before a consonant sound (the boy, the house), [ði] before a vowel sound (the egg, the hour). Native speakers, however, sometimes ignore this distribution, in particular by using [ðə] before a vowel (which is in turn usually reinforced by a preceding ʔ), or by using [ði:] in any environment, though especially before a hesitation pause. Furthermore, some speakers use stressed [ðə] as a strong form, rather than the usual [ði:].
My question is: when native speakers use [ðə] instead of [ði] before a vowel sound, do they do it on purpose or accidentally? If it is on purpose, how do they (typically) decide which pronunciation to use? What is a valid reason to use [ðə] before a vowel sound?