Firstly, thanks a lot for asking this question as it gives me a rare chance to discuss my rather obscure interests in this sub.

As for whether or not Sikhs are over-represented as a percentage of Indians moving abroad today, the answer is mostly no, but, in accordance with the 20-year rule, I won't dwell on that very much.

Nonetheless, you are correct in your identification of the Sikh diaspora in Canada as being disproportionately large when compared to other immigrant Indian populations in the country. As for why the Sikh population in Canada is relatively sizable, the answer is twofold- compared to similar groups, Sikh immigration to Canada began quite early, and early Sikh immigrants established important socio-political connections in North America that bolstered later immigration patterns. One important thing to keep in mind throughout is that while the Sikh population may be small as a portion of India's total population, the total global Sikh population is quite sizable, at about 25 million (about twice the size of the global Jewish population, for reference).

We can broadly split Sikh immigration to North America into two time periods. The first "wave," spanning the early 20th century, was comprised of mostly unskilled workers motivated to emigrate principally by economic conditions. The second "wave" came following Indian independence and represented a wealthier better-educated group motivated by both economic and educational purposes. This arguably represents the present condition of Sikh immigration to Canada specifically, but an in-depth discussion of modern immigration patterns isn't relevant to this sub. Within this second wave was a brief influx of refugees fleeing political violence in Punjab during the 1980s. I shall discuss this influx briefly at the conclusion of my answer, but the impact of this group was mostly upon the culture of the North American Sikh diaspora, not the numerical population.

Troubles in Punjab and Sikh Tours of Duty

By the 1870s, the Punjab (the homeland of the Sikhs, located in Northern India. Today split between India and Pakistan) had begun to experience financial strain. The significant extension of canal and irrigation infrastructure by the British had increased the yield per acre of Punjabi fields prodigiously, leading the price of land per acre to skyrocket nearly tenfold. Nevertheless, agricultural yields remained unpredictable, and repeated shortages in 1869 and 1877 damaged the financial stability of local farmers. This resulted in the rise of a land speculating industry never before seen in the area, with newly established moneylenders charging heavy interest rates, plunging already struggling farmers further into debt. Compounded issues even further, the population of the Punjab had risen almost 20 percent, causing urban overcrowding and leaving some families with very little land upon which to subsist (land was usually split evenly among sons, reducing even previously considerable holdings to mere plots in a short period of time). An epidemic of bubonic plague (which would continue to break out sporadically until well into the mid-20th century) only added to the farmers' woes.

Partially attempting to solve these issues and further intending to increase agricultural revenue for the Raj as a whole, a program of colonial settlements along newly constructed canals was undertaken in the 1890s (these projects had existed prior to the 1890s as well, but were slow to develop). Undertaken in the Rechna Doab primarily in the Llalypur and Rawalpindi districts, these settlements opened previously "desert" land to new agriculture, displacing the indigenous inhabitants (who were considered less suitable for agriculture) with inhabitants from the Bist and Bari Doabs. A map of Punjab may prove helpful for those unfamiliar with the area's geography. I have attached a Wikimedia link to one as the first link in my sources. The Bist and Bari Doabs contained the majority of the Sikh population at the time (post-partition, the Sikh population is now concentrated in only the Bist Doab).

While these projects lessened the financial strain upon part of the population, outrage soon developed over the government's heavy-handed administration of the new land allotments. Colonists were prohibited from making wills regarding the land (a long held right throughout the area) and were not allowed to clear-cut trees. While failing to enliven the public sentiment or significantly relieve the strain upon impoverished agriculturists, the canal colonies did provide Sikhs in the Punjab their first experiences with immigration (albeit only across a small distance), proving the possible financial benefits of relocation. The dissatisfaction of those that did manage to secure a land allotment provided an additional impetus for immigration.

Contemporary to these upheavals in the Punjab, Sikhs serving in the military and civil service also got their first tastes of long-distance displacements. The conquest of Burma resulted in the deployment of a great deal of Sikhs in Southeast Asia for military and peacekeeping purposes. Sikhs were also sent to China and Hong Kong during and after the Opium Wars (designated as a "martial race" for their organization during as the Dal Khalsa and their loyalty during the Sepoy Mutiny, Sikhs were recruited into the Raj's army in significant numbers; other groups were also given the same designation). Thus, minor Sikh enclaves emerged in East and Southeast Asia, providing future immigrants key layover areas and providing within the Sikh community as a whole a general familiarity with immigration.

Arrival (and brisk stoppage) in Canada

Early Sikh immigrants to Canada sought economic opportunities in the resource industries of British Columbia. Sunder Singh, a contemporary Sikh immigrant to Canada and Sikh intellectual, theorized that Sikh troops returning to India via Eastern Canada from Victoria's Diamond Jubilee may have popularized the destination among their countrymen. Western railway companies also recruited from Sikh enclaves in Hong Kong, whose employment later attracted impoverished Sikhs directly from the Punjab. In any case, Hong Kong quickly developed as a hub for Sikh immigrants to North America (who usually traveled to Hong Kong before taking steamships to the North American West Coast).

Already disaffected by large scale immigration from China and Japan, white British Columbians did not respond positively to Sikh immigration. The Vancouver Sun summed these views in 1907, writing, "we shall have these Asiatics swarming over here in hundreds and threatening to defeat forever all hope of making this a white man's country," noting that their assimilation, though impossible, "would destroy the Canadian type mentally and physically." That same year saw anti-Asian riots in the city (Sikhs mostly escaped these riots as they comprised only a small portion of the total immigrant community).

Attempting to quell the issue at its source, Canadian officials appealed to Lord Minto, Viceroy of India, to impose a tax of $200 on Indian immigrants to Canada and discourage avenues of emigration from Punjab. Minto, however, preferred for any restrictive legislation to come from Canada, for fear of aggravating Indian politicians.

Left to their own devices, the Canadian government in 1908 passed a series of laws severely restricting Sikh immigration, effectively arresting it entirely. The first law made true the initial suggestion made to Lord Minto, requiring every Asian immigration from a country without treaty rights with Canada to possess $200 at their point of entry (a respectable sum for the average Sikh immigrant). The second prohibited travelers from entering Canada unless they came directly from their country of birth via a "continuous journey." Sikh immigrants, lacking considerable funds and wholly reliant upon their Hong Kong layaway, proved incapable of surmounting these new laws.