kpj, what do you mean by over reaching? Do you simply add sets and reps to your current workout? Or do you workout more during the week?
Basically, doing too much. Volume changes week to week, normally with the third week being really hard work. It's not anything drastic i.e 2 or 3 sets extra on a big lift and some assistance exercises.
For example, 4 weeks may look like this
Wk1: 5 x 3
wk2: 5 x 3
Wk3: 7 x 3
Now, "5x3" should very challenging, and the prospect of another 2 working sets should not be a nice one.
On assistance, i'll tend to add another set to the important stuff, so, if was normally 3 x 8, on week 3, it'll be 4 x 8.
Also, I love speed work, and I program in heavy singles after the speed work on the third week, too, for those that do speed work, anyway.
I think consistently improving on a big lift at a low rep range for 3 weeks will burn you out to some extent, anyway, eve if you don't add more to any of those 3 weeks. I'm sure Louie Simmons said you can't get stronger for more than 3 weeks in a row. I've actually found this to be very accurate (beginners aside, obv). There is "something" about 3 weeks of consistent strength progress that makes your body want to chill out for a while and recover. Add a few more sets into the mix on that third week and tip yourself over the edge just a little in terms of what you're normally capable of recovering from.