Feature, not a glitch. This was added in the last content update. From the wiki:
"On hard difficulty, zombies can spawn additional zombies to “help” when damaged. Each zombie has a "likeliness to call reinforcements" statistic which ranges from 0–10%, and "leader" zombies (0–5% depending on regional difficulty) get a bonus of 50–75 percentage points to the stat. When the zombie is damaged by an entity or is damaged while targeting an entity, up to 50 attempts will be made to randomly choose a spawn location (0 or ±7–40 blocks away in all three axes) that is above a block with a solid top surface, has light level 9 or lower, has no players within 7 blocks, and has no colliding entities or blocks at which to spawn the reinforcement. Both the damaged zombie and the new zombie will have a 5 percentage point penalty to their "likeliness to call reinforcement" stat, making it less likely that an infinite number of zombies will spawn."