Pitch Accent Confusion

Occasionally, I hear the pitch accent of phrases that should be heard as 平板型 end up not being this way. Some examples are 卒業して、それは、これは、気がする、私は、私が. All of these words and phrases should be "flat". However, I have heard a couple instances where the pitch drops, such as in それは{LHL}、これは{LHL}、そつぎょうして{LHHHHHL}、きがする{LHHL}、わたしは{LHHL}、わたしが{LHHL}. Does anybody know what exactly is going on with these examples?