[daeron80]

That's just the way it works. There are two reasons I know of. One is that filters shift phase, and new phase relationships in complex material are likely to cause some peaks to be higher, some lower. The other is that low frequency waves carry high frequency ones. If a sharp transient happened to be sitting in a trough, it might not clip; remove the low freq trough, and tink!

The solution for future tracks is to leave headroom. A good 12 dB. Your mixes will sound better, not just because it eliminates clipping, but for a host of other reasons. One biggie is that you won't be worrying about not clipping, so the mixes will go quicker and more intuitively. Another is that A/D converters start to run out of headroom when pushed hard. They just sound better if your peaks stay down a dozen dB.