Why does a buffer amplifier change the behaviour of a bandpass filter?
One of the most important uses of buffers (being them emitter followers, voltage followers or whatever) is that the isolate the stages presenting a stable impedance on both input and output.
Since a passive filter is influenced by these impedences (RF people fight with this all the day) simply chaining them has the first one seeing the input of the second one and the second one seeing at the input the output of the first one. These are complex impedances and the result is… well, not immediately obvious. That being said ladder filters are designed with that in mind, but as a single unit.
A buffer in the middle present to the first stage a (usually high) fixed impedance and to the second stage a (usually low) impedance. In this way you can design both stages independently and 'glue' them with a buffer.
This is valid not only for filters but for signal stages in general. There is a whole two port network theory based on this chaining.