I'm not a break type rider although I'll try and create/join them for kicks.
However, when I see breaks go up the road, it's usually for a couple reasons.
In Cat 3 or lower races the field gets totally strung out due to course features (corners, narrowness, wind, weather). It starts to become very difficult to find shelter. In the 3s and below usually you'll find a wider range of fitness/ability than 2s and above so as the field strings out you'll get some gaps etc. The strong riders can launch and no one will get them.
The other type is when you get someone unknown, say a strong triathlete or runner or mountain biker who goes up the road and there are a few guys that train with him and know him and they go with him. Now you have one guy in a non team kit ("Holy cow that guy in the blue was so strong!") and a few guys in team kits that you don't consider a threat individually. The unknown guy does a Bernard Hinault pull (when Hinault bridged to a break he'd drill it for a few minutes, build a minute or two buffer, then pull off and assess the situation), the break suddenly has 50 seconds, and everyone starts looking at each other. Even if a Cat 3 team gets together to try and chase they usually do it wrong, they aren't strong enough, etc.
In Cat 2 or higher (or P123 for example) the fitness/ability levels are more consistent. I watched one race, 50 laps P12 I think, and it was strung out for 49 of those laps. I had no idea how anyone could get away, but in that case one guy took off and held off a hard charging field for virtually the whole race (45 lap solo? 47 laps? I'll ask him next time I see him). Two guys in the field were good pros, Jeff Rutter and Graeme Miller (racing for Scott BiKyle). They couldn't catch him. So that's one type of break, where someone commits and can TT at 28 mph slow and 32 mph fast. His tactic was to go 28 mph and if he heard me say they were chasing or the gap came down he'd go 30-32 mph for a lap or two until the gap stabilized again, then he'd slow down to 28 mph. Sounds so simple when he says it. I blew myself up going 30 mph for a lap - he did a similar effort for 40 something laps.
The other thing that happens is it's like the pros in the Tour and a break suddenly gets 5 minutes in 10km. The right combination goes up the road. In a P123 race last year three separate strong breaks went up the road. Each time the field would gather itself up and guys would start launching off the front bang bang bang, and after 4 or 5 or 6 laps the 30 second break would come back. When the 3rd break went the Livestrong kid missed it, every guy who had a friend or teammate up front sat up, and suddenly we were going about 18 mph on the backstretch. The break quickly gained 40-50 seconds.
Then, kind of ironically, very close to the end, the Livestrong kid blasts out of the field solo, catches the break on the last lap or so, and wins the sprint. Haha. Very impressive. So he did the "guy who breaks the field" kind of ride to counter the "break that gets blocked off the front" kind of break.
I just realized my two examples are on the same course.
As a disclaimer I've been in (very unusually for me) 4? breaks this year, 2 of them launched either at 3 to go or 2 to go. I blew myself up in each race-end attempt and actually shelled myself in an early break (caught/dropped). That's my recent history in breaks and even when fit it's not much different.