A review of chromatography fundamentals tells us which parameters effect pressure. It pays to address the basics when troubleshooting a problem.
For static isocratic conditions, a reduction in initial column or system back pressure would most likely be caused by changes in the:
(1) ACTUAL Flow Rate (Measure it with a volumetric flask to see if it is lower than the set flow rate. HPLC systems do not measure flow rate. They simply pump at a preset speed). Perhaps you have a leak which has changed the actual flow rate?;
(2) Temperature increase (col compartment or room temp is warmer);
(3) A change in the mobile phase composition due to evaporation, different preparations of the "same" mixture (different people, techniques or methods) OR a defect in the mixing valve resulting in an unexpected composition change. Lower total viscosity equals lower back pressure.
(4) Poor quality column re-equilibration. If you fail to operate the and/or run the method in a consistent way, then variations should be expected.
(5) A change in the physical packing of the support. This is commonly seen in UHPLC applications run at very high pressure (>8,000 psi), which are very rare today. However, if the column is newly equilibrated in a solvent system which differs greatly from the type it was initially packed in, it may change its internal distribution resulting in small changes to the observed back-pressure (the fines could move in position, the overall porosity could change). If true, this should stabilize over time so a log should be kept.
PS a change of 10% in pressure after 1000 injections is probably nothing to worry about!