Senna Pauses up to 15 Seconds

I saw someone else complain about this, but couldn't reply to their post, for some reason:

In the match I just played, I had a Senna as the first champ. In at least three different rounds, she suddenly slung her gun on her shoulder and stood still for a very long time, like five to fifteen seconds.

In each case, she did not have a broken sword symbol for Swordbreaker, did not have a swirl for being stunned, and I saw nothing happen to her that would **immobilize **her.

She was **within reach of enemies to shoot, and had a completely open path **if she needed to move closer, each time.

She was, if it matters, standing on a water element patch, so she fired her ult immediately at match start. I think she may have slung her weapon and stood still during/after the second or third ult, though, never that first one.

The first time she stopped for 15 seconds or so, she had nothing but HexTech GunBlade on. And the water elemental patch. The second and third times, she also had Guinsoo's RageBlade.

The other post on this problem had several upvotes, so I suspect I'm not the only one who's encountered this.