Data analysis question: More detections closer to set-up date

Dear friends:

I am using camera traps to investigate wild boar population density and I am analysing data collected over 7 weeks. I broke them into four 12 or 13-day sections (pseudo-replicates) and realised the number of detections in the first section is much higher than the other 3 sections (216, 147, 145,124 respectively). May I ask has anyone come across similar situations (i.e. the number of detections being high immediately after set-up, and decreases after the 7-10 days or so)? What could be the possible reason for this pattern?

Thanks in advance