Trundle's Q vs towers

I was playing a match as trundle earlier and when I was hitting attacking the top first tower, I used my Q at one point for the auto reset (I know it doesn't work on towers anymore) and while his Q was still active my damage to the tower decreased not once, but twice

Before I used it, I was doing about 100 damage per auto to the tower, then when I used my Q it went down to 79 for a few hits, then it went down further to 55 for the rest of the duration.

It seems like it's the same thing that happened a bit go with Kassadin's W active