It's an example of assimilation, which happens when sounds modify each other when they are sounded one after the other. There are two common sorts. The first is anticipatory assimilation, when the first sound changes because of the following sound - the final /t/ in 'that' becomes a /k/ before /g/ in 'fat girl', for example. The second, shown in your examples, is coalescent assimilation, when the two sounds combine to form a third, different sound.
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