Check the salinity and water contamination. It might be due to blocking some pore throats. There is always fracking by the way. It might not be due to pressure, but thermal fracking is mostly there due to contrast in temperature between formation and injected water. Did you increase the water temperature during the injection process? Also, like Dr. Hassan asked, are you injecting under fracturing conditions? If you are, then any decline in pumping pressure will result in closing these fractures and thus decline in rate and increase in bhp occurs. I am assuming you saw some decline in injection rate. If you do not, then it might worth it to check your instruments. I highly recommend you start using Hall Plots for your injection. Once you generate the Hall Plots for your wells, try to check if the issue is with one well or all wells (I am assuming you are using central facilities water injection), then at least you will start realizing if the issue is with the water or the well. Also, for each well decide if the decline is abrupt + skin or gradual + skin. If gradual for a well then you might want to check your well for scaling .. etc issues that result in decline in pressure and thus injection rate .. etc
Good luck
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