The AI bumps and pushes you all the time, they have since as far back in the series as I can remember. It actually got a little bit better in GT5 as it used to be quite common for them to get along side you and simply plow you off the side of the road all the way down a long straight. That's become more rare but they still constantly ram you under braking and mid corner, and shove you off-line or off-track in corners. This happens even if you make every attempt to avoid contact, coming in clean and early with plenty of room, staying tight to your side of the road and even hanging a couple wheels off the inside of the track to give them racing room.
I don't think they do it to be intentionally dirty, I think they are just really stupid and manage to frequently forget that you are there right in front of them. Then again, the very slow and actually very dirty driving line they are intentionally taught to drive around almost every track might suggest that PD isn't above making them dirty and dangerous on purpose. If you do take their stupid line into account and give them room to turn in way too early for the corners and swing out too wide too early it can help but still doesn't eliminate the issue.
One thing that I find quite odd about it is that once they "forget" about you and decide to drive right into you, they almost never give up until they've forced past you or forced you to spin. They only seem to "remember" if you can shove back hard enough to make them make a correction and then they back off.
On those occasions when you can get side-by-side with them driving clean, they can actually be quite fun to duel with as they aren't that bad at adapting to a compromised line. Sometimes you can stay 2-abreast for several consecutive corners before you make a mistake or they chicken out. You do have to be wary of getting out of control however, as they often won't be smart enough to give a little extra room if they see your rear corner sliding towards their front corner and you'll spin.
While they do drive slightly more aggressively overall in the later stages, I think they only seem to be dirtier since the higher-end cars are much less resistant to the little nudges you can correct and drive away from in slower, heavier cars. Most players also probably wobble around away from a perfectly smooth line a little bit more in the faster and/or twitchier cars which compounds the issue.