Because your brain is sensitized towards sexual stimuli, any and all sexualized imagery you see or thought you have can drive your brain crazy. A LOT of rebooters have the experience of just watching TV (for example) not even thinking about porn, then an ad comes on that has a woman in a bikini on a surfboard (or something like that) and then all of a sudden they get STRONG cravings to use porn and masturbate. Afterwards they have a feeling of 'where the heck did that come from?'. The reason why is because their brain is still sensitised towards their addiction and so is easily triggered to give in to it.
Does it get better over time? Yes. But it can take months or a year or more of complete abstinence to get fully better (that doesn't mean there won't be improvement before that time, that's just how long it takes to get fully better). And as discouraging as it might be to hear this, it's likely that we (ie. recovering porn addicts, I'm in the same boat as you here) will never fully recover from this kind of sensitization to sexual material. Studies have been done on former drug addicts that have shown that even after 20+ years sober, their brain is still sensitized towards seeking out their old addiction ? if you show them pictures of white powder for example, their brain will still light up in an FMRI scan, whereas the brain of someone who has never used will not. There is huge improvement over time of course, but it looks like the brain never fully heals.