Yes, it looks like the July 18th release date of Plants vs. Zombies 2 is going to have to be pushed back until later in the summer. A particular reason has not yet been given, just that developer PopCap want to "make sure the game lives up to fan's expectations".
Pushing back a release date can do a lot of good, as it gives the team time to remove bugs, clean up the programming for a smoother experience, and put an all round polish on their release.
This highly anticipated follow-up is not going to lose its potential audience by due to a one month delay, especially when that delay could be beneficial to the overall quality. So sit tight, and look forward to playing a (hopefully) bug-free game later this summer.