Pistons extending upwards break sand and gravel the same way a torch breaks falling sand and gravel.

I have included a world download with a machine to demo the behaviour. The bug is most distinct with the central repeaters on the second setting, but the first setting will result in the block breaking eventually. To start the machine, load sand or gravel on the pistons and press the stone button.

Please note, this may be (or may not be) related to bug MC-46225, but this is different behaviour. In snapshots up until 14w10c the piston disappears, but from 14w11b the piston no longer disappears, but now the block breaks as described above.

Further info - if you replace the pistons with sticky pistons in the test world attached, the problem will not manifest until you change the repeaters to the first setting. On that setting the behaviour of sticky and non-sticky pistons is identical.