Happened to me on a number of flights, just in the last month at least 4 times, all on E-145 segments, and on two different segments on two consecutive days this week. Usually seems to happen around 2-4 days before departure, invariably the computer moves me from the E-145 exit row or seat 1A to one of the last rows in the plane. Can't make a change that will stick on-line, always requires a phone call to get my original seat back. No change in equipment unless there are multiple types of E-145 with identical seat maps. Nothing like this happened to me prior to 3/3, now it is the new normal.

They hang a tiger's food from a tree in the zoo to make zoo life more challenging for the tiger, maybe this is United's way of making flying more interesting?