Why would an aircraft drastically reduce in speed and then accelerate during landing?
Your question uses the adverb "drastically". It's very unlikely that whatever happened was drastic. When an airliner does something drastic, it's a pretty memorable experience. In the absence of visual cues, some other sensations can be magnified in our experience, so it's quite possible that any change was less extreme than it seemed to be.

Sometimes, the plane is actually slowing down and speeding up. This could be in response to an air traffic control instruction, a not-quite-right adjustment that needs to be corrected, changes in local wind condition and so on.

It could also be that the plane needed a little adjustment after intercepting the glide-slope.

At this stage of its flight, the crew many need to respond to many different kinds of factors in its environment.

However, just because you had sensations of speeding up or slowing down doesn't mean that that is actually what happened. Changes in pitch can also produce these sensations, especially when combined with changes in engine sound.

In short, if that did actually happen, it could be for all kinds of reasons, but just as likely, it was an illusion.