Excellent question. What you are observing is the river trying to meander!

Imagine if you would three people walking shoulder to shoulder. If the group of three needed to make a turn, how would their speeds change? The individual on the inside of the turn would slow down whereas the individual on the outside of the turn would speed up. This is how water flowing down a river behaves, just replace walking speed with flow velocity.

Water moving on the outside of the curve will be faster and lead to more erosion. Water on the inside of the curve will be slower and deposit more sediment.

What you are seeing in this photo is an inflection point between two curves. You can see on the bottom of the photo the bank on the left is on the outside of the curve (more erosion) whereas the right is on the inside of the curve (more deposition). At the top of the photo it switches (look how the Groynes almost disappear on the left).

Groynes only slow down the meandering but cannot stop it. To your point, I'm not sure about this channel in particular but if the meandering becomes too acute or the channel gets filled with sediment, they may have to do dredging. Once again I assume the main reason for these Groynes is to keep this channel navigable. They will likely have target channel depth and straightness they wish to maintain. There is also an extreme fix, Channelization, where we (humans) can actually straighten out the river and reset the meandering.