PRNG bug for sure or a hack. Its not possible to generate the same priv key for a random address that was used, this might be a PRNG bug that actually made that event possible or someone has hacked the way onto your cellphone.
Not revealing the wallet is a really big mistake and unethical thing to do.
An unethical dev could also insert code in the wallet that would occasionally "generate" an address that he already has the privkey for. These might be pre-made and hardcoded into the walletsoftware, so there wouldn't even be signs of nefarious network traffic, or previous usage of said keys. The only way to be reasonably sure the wallet software is legit is to review it's source, then compile it yourself. In general with mobile wallets, don't store more BTC than you can afford to lose.
On the other hand, it's also important to present proper evidence, as anyone could really make an accusation against any wallet-developer. But if the wallet developer chose to, he could make available the source code for review, which would prove without doubt that the source code does not contain anything nefarious. Of course there could be malicious code inserted in the distribution process by a party with the necessary resources to do so.
I am not very familiar with deterministic builds myself, but here's an article about it:
http://www.conifersystems.com/2008/10/17/build-determinism/Basically, the same input, should give the same output, so interested parties can run sha256sum on the binaries, to check they're legit, also checksums can be signed by the developer. If you download an app from any appstore, and there's no such security mechanisms in play, how can you be sure the software is legit?
Also, it's worth noting that for a very popular mobile wallet, if only a very low percentage of users are hit with theft, the overall negative impact will probably not be very big, so for the unethical dev, this brings a small but steady income stream.
Personally I think all possible details should be disclosed to the community as early as possible