It's been a great year so far for big tech.

Since the calendar flipped to January, the ever-popular SPDR Technology Select Sector ETF (XLK - Get Report) has surged more than 30% on a total returns basis, besting the stellar 18.7% return in the rest of the S&P 500 by a massive margin.

But while big tech continues to lead in 2019, not all tech stocks are looking equally strong here. In fact, one of the biggest tech titans just started waving a red flag in its price action this fall.

To figure out how to trade it from here, we're turning to the chart for a technical look.

The massive tech stock that's showing cracks this fall? It's Alphabet  (GOOG - Get Report) (GOOGL - Get Report) :

Make no mistake, Alphabet isn't teetering on the edge of oblivion by any stretch of the imagination. But it's also not leading the pack. Since the start of 2019, Alphabet is up about 15.9%. That's strong performance on an absolute basis, but it's a far cry from the rest of the tech sector - and a miss vs. the S&P 500, for that matter.

More concerning, shares triggered a pretty clear-cut breakdown when the calendar flipped to October.

It doesn't take a technical trading whiz to see the well-defined uptrend that's propelled shares of Alphabet higher since early June - or the breakdown below the bottom of that trading range that shares experienced last week.

Alphabet's failure to catch a bid at trendline support is troubling because it opens the door to more downside risk in a stock that's already been unable to keep pace with peers.

That's clear from relative strength as well, as it has essentially been flat since the middle of the second quarter. This signals that Alphabet is more or less even with the broad market from a relative performance standpoint over that period. In a market where owning leaders is crucial, Alphabet isn't one.

Failure to participate in a broad sector rally is a big problem for Alphabet investors here.

It's important to remember that when you're allocating to specific stocks in a sector, it's a two-way bet: You're deciding to own a collection of stocks, and implicitly, you're deciding not to own a different collection of stocks. Looking at big tech alternatives like Apple (AAPL - Get Report) right now, it's clear that Google should be part of the latter.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.